Friday, May 31, 2019

Zip cord dipoles. Any good?

My first instinct when I see a long piece of two wire electrical lead (or zip cord) is to pull it apart and use the wire. Often the wire is nice and thick and good for HF dipoles.

However some people only pull some of the cord apart, using the remaining together section as a feedline.

This sounds a bit crude. The cord's impedance is a long way from 50 ohm. And there can be significant losses, particularly towards the upper end of the HF range.

You may need an antenna coupler between the transceiver and the feedline. But with any luck the types built in to some transceivers, only good for a limited impedance range, may suffice.

I've never used a zip cord dipole myself. But here's some experiences from others with them:

* K1TD QST article on zip cord antennas (pdf)

* KE8DI zip cord antenna

*Zip cord portable receiving antenna

If you've succeeded (or failed) with zip cord antennas please let me know in the comments below. 

PS: Enjoy the Daily Antenna and my videos? Wish to support them? If so please store this link to Amazon Shopping as a bookmark or favourite. Then when you buy something I receive a small commission (at no extra cost to you). Or browse my books page to see if any titles appeal. 

Thursday, May 30, 2019

An RF current indicator for antenna tests

A handy little instrument for antenna tests is this little RF current indicator here. You can move it near antenna elements, counterpoise elements and open wire feedline to identify points of maximum current. This gives you some reassurance that things are working as the theory books say they should. 

It's so simple that you don't need a circuit. The parts you need are as follows:

* Tinned copper wire - about 30cm long for sense antenna
* Germanium diode as used in crystal sets (eg 1N34A, OA95 etc)
* Disc ceramic capacitor - anywhere from 1nF to 100nF
* Analogue meter movement - anywhere from 50 uA to 1mA full scale deflection

Both the diode and meter are polarised. The cathode of the diode (end nearest the thin stripe) must go to the positive (+) on the meter. Meters are marked with + and - terminals.  I used a small printed circuit board on the back to provide some extra rigidity. Use a hacksaw to saw a slot to separate both connections of the sense loop. 

To use the indicator run its sense wire parallel with the antenna element or feedline to maximise coupling and get an indication on the meter. Move it a little away if the meter reads full scale. Move it up and down the antenna or counterpoise wire to see how the current varies with the meter's position along it. 

PS: Want to read more about antennas? Consider this selection of antenna books. They are affiliate links meaning that I receive a small commission (at no extra cost to you) if you decide to purchase.


Wednesday, May 29, 2019

The inverted-L

One of the simplest antennas for the lower HF bands is the humble inverted-L. Take a pole, run a wire up it and tie its end to another support 20 or 30 metres away. It's fed via an antenna coupler (such as a pi or L-network) at the bottom. Clip onto a nearby metal object for a counterpoise, preferably via a ground tuning unit. Then you're on the air, with contacts possible on multiple HF bands. 

It's great if you want to work anyone - near or far. In that regard it contrasts with entirely vertical antennas like the ground plane or tee that are great for DX but not your middle distances just beyond groundwave coverage. Hence you might pick an inverted-L for a field day, national parks, SOTA or local net operation.  Whereas a ground plane might better suit DX operating. 

The Inverted L has been my preferred antenna for portable operating due to its simplicity. Lengths are not unduly critical. But on the lower HF bands if you want vertical polarisation handy for DX performance make as much of it as possible vertical. Or if you want a mix keep it at about half vertical and half horizontal. But don't worry if your supports are low and about two-thirds is horizontal - you will still get contacts. 

A total wire length of near 1/4 wavelength on the lowest frequency of operation can work if you have a good ground. Otherwise nearer 1/2 wavelength is better.  However exactly 1/2 wavelength means extremely high impedances. That can present high  feedpoint voltages and difficulties with some antenna couplers.  So you might want to go a few  metres longer or shorter than 1/2 wavelength to avoid these extremes.

A bit over 20 metres is a good general purpose length for casual operating on bands like 40, 20 or 30 metres. Although I'd suggest (say) 35 or 45 metres for better results on 80m.  

There are thousands of happy inverted-L users around the world. Read about (or watch) some of their experiences here:

* W4RNL's all band inverted L
* The Inverted L antenna and NVIS 

You're invited to comment below if you've had experiences (good or bad) with the inverted L. 

PS: Enjoy the Daily Antenna and my videos? Wish to support them? If so please store this link to Amazon Shopping as a bookmark or favourite. Then when you buy something I receive a small commission (at no extra cost to you). Or browse my books page to see if any titles appeal. 

Tuesday, May 28, 2019

The pi network antenna coupler

Want a simple to build antenna coupler good for a wide range of impedances? A pi network may be the thing for you. Simple to build it's ideal for a variety of unbalanced antenna including end-fed wires. Both variable capacitors in it are earthed. This makes construction easier if you're using a metal case. Another benefit is that it forms a low pass filter. That means it suppresses harmonics. Just the thing for use with a simple QRP rig that might only have one filter section! 

Pi networks for QRP can be built using a tapped coil on some plastic pipe or the inside of a kitchen paper roll. Or maybe a tapped toroid. Either use a rotary switch or aligator clip to select different inductances.  Plastic variable capacitors can work for low power (up to about 200pF for 7 MHz or 400 pF if you want 3.5 MHz coverage) but if you have them an air spaced metal type is preferred. Use a switch and a fixed capacitor (equal to the variable capacitor's maximum) to switch in more capacitance if required. Special transmitting variable capacitors should be used for high power. You'll find the parts at any good hamfest. 

Here are some practical pi network antenna coupler designs to get you started.

* KC5LDO Pi network tuner
* AA3SJ 50 MHz pi coupler
* WF7I pi network
* DJ0IP pi network (and others)

If you've built a pi network coupler please share your experiences in the comments below.

PS: Like something to read? Read worldwide and favourably reviewed.  Available in both electronic and paperback.

Monday, May 27, 2019

Ground tuning units, counterpoises and small antennas

If you have a fence or pier with a metal railing nearby, why not use it as part of a counterpoise system? That lessens what you need to carry and you can get some great results.

It's a good idea to have a ground tuning unit and a relative indicator of current so you can adjust it for maximum. Even if you don't have much height you can sling a thin wire between two fishing poles and get some very good results even with surprisingly short antennas. For example a 5m vertical with a 10m flat top (for a capacitance hat) can work very well on 3.5, 7, 10 and 14 MHz.

The video below has further information on building a ground tuning unit.

Want more information? There was an article in Practical Wireless October 1990 on building a Ground Tuning Unit. Some other notes are below:

VK2HAS notes
More detail on the above
* Video by VK6OP

If you try any of these, please let me know how you go in the comments below.

PS: Want more portable antenna ideas? Many are in Hand-carried QRP antennas. Thousands sold. Favourably reviewed. Available in electronic or paperback. Find out more here

Sunday, May 26, 2019

A top loaded vertical over salt water

Can't achieve much height but want something good for bands like 1.8, 3.5 or 7 MHz? If so there's much to be said for a top loaded vertical. Especially if you can position it over salt water and have a metal railing that you can use as a counterpoise.

Such verticals can really pull in the DX. Band noise even sounds different on them. The video describes a very simple vertical than can be loaded up to operate on several bands. It's main disadvantage is you need two rather than one pole. But they don't need to be very tall and they can be light as the antenna doesn't weigh much.

The other thing you'll need is a ground tuning unit and RF current meter. The one in the video is as crude as can be but there's ones on the web that are more elegant. They're a simple afternoon project and are recommended for the portable antenna experimenter. 

The antenna I tried is just 10 metres across the top and 5 metres tall. Yet it does quite well on 3.5 MHz. Watch the video below for further information. 

PS: Want to get more from amateur radio? This book can help. Available in electronic and paperback form, exploring the facets suggested will keep you entertained for hours. Find out more at vk3ye . com or search the title on Amazon. 

Saturday, May 25, 2019

Triple Six multiband delta loop

Got 18 metres of wire, a couple of poles and not much space? If so this vertically polarised delta loop might work for you. It operates from 10 to 24 MHz and can give DX contacts. But is it directive? Watch the video and find out.

PS: Want more portable antenna ideas? You can't go past Hand-carried QRP Antennas. Read the reviews by searching the title on Amazon or find our more here. Available in both electronic and paperback formats. 

Friday, May 24, 2019

Rowland Ring antenna - WSPR transmitting tests

More Rowland Ring antenna experiments - this time transmitting WSPR while loading up a goal post (or a flag pole).  Results are mostly inferior to my full sized antenna at  home, but not always.

PS: Enjoy the Daily Antenna and my videos? Wish to support them? If so please store this link to Amazon Shopping as a bookmark or favourite. Then when you buy something I receive a small commission (at no extra cost to you). Or browse my books page to see if any titles appeal. 

Thursday, May 23, 2019

How to calculate a mast's height without having to climb it

Just a sunny day and tape measure needed. No trigonometry or complex formulas.

PS: Enjoy the Daily Antenna and my videos? Wish to support them? If so please store this link to Amazon Shopping as a bookmark or favourite. Then when you buy something I receive a small commission (at no extra cost to you). Or browse my books page to see if any titles appeal. 

Wednesday, May 22, 2019

The beverage antenna for low band receiving

Have a long narrow yard? Want the best possible reception of long distance AM broadcast, 1.8 and 3.5 MHz amateur band signals? If so a beverage is the antenna for you. If you can't hear them you can't work them and a Beverage will give you an edge over those using their noisy vertical for receiving as well as transmitting. 

The main thing you need is wire. Lots of it. Eg a wavelength on its lowest operating frequency. If it's some solace though, the one Beverage will work on several bands. The other key parts you need are a terminating resistor (470, 560 or 680 ohm) and a ferrite toroid to match the antenna to your receiver's 50 ohm. Beverages are directional, with maximum response roughly off the terminating resistor end. 

The ferrite is at the receiver end. The receiver is connected to the primary winding. One side of the secondary winding is earthed while the other goes to the Beverage's antenna.  At the other end the Beverage wire connects to ground via the terminating resistor. Great height is not required. Even an old fence (with timber posts and a remaining top wire) could be used. 

Get a good drink and read more on Beverages here:

* Steve Whitt Build a basic Beverage
* W8JI on beverages
* AA3PX beverage design
* Advanced beverage antenna designs (pdf) 

PS: Enjoy the Daily Antenna and my videos? Wish to support them? If so please store this link to Amazon Shopping as a bookmark or favourite. Then when you buy something I receive a small commission (at no extra cost to you). Or browse my books page to see if any titles appeal. 

Tuesday, May 21, 2019

A 6 and 10 metre dipole for summer

For those in the northern hemisphere now is the ideal time to build a 6 & 10 metre dipole. Why? Because the summer sporadic-E season is about to start. You'll be able to make solid contacts up to about 2000 or 3000 km with just 5 watts of SSB on 6 and 10 metres.

Here's the dimensions for one made of thin wire.

Take it to a nearby park on a nice day and rack up the contacts.

More details, including construction information and a demonstration in the field, are in the video.

If you build one please let me know how you go in the comments below.

PS: Ever find a term in amateur radio you don't understand? The Illustrated International Ham Radio Dictionary can help. Available as an ebook for $US 5 with paperback also available. Details in the link.

Monday, May 20, 2019

Rowland Ring Railing HF receive antenna

A few days ago I loaded up a football goal post as an HF transmitting antenna. I made a couple of contacts on HF. Coupling was via a Rowland Ring. This time I'm using one with a pier railing about 40m long.  It's quite a good HF receive antenna, with quite low noise (even though homes and shops are a few hundred metres away).  More in the video below. 

PS: Enjoy this blog? Wish to support it? Please store this link to Amazon Shopping as a bookmark or favourite. Then, when you buy something I receive a small commission (at no extra cost to you). 

Sunday, May 19, 2019

Don't trust all SMA - SO239 adapters

Yesterday we talked about the differences between BNC connectors and how using the wrong type can make connections intermittent. Another connector problem, particularly with VHF/UHF gear, is SMA to SO239 adapters. You might think a problem is with your transceiver, antenna or feedline, when it's actually the connector. And it can be difficult to track down. Bargain-hunting eBay buyers beware! More in the video.

PS: Returning to amateur radio after a break away? If so Getting back into Amateur Radio is the book for you. Favourably reviewed. Low price. Choice of electronic or paperback. Find out more here.

Saturday, May 18, 2019

Not all BNC connectors are equal

Signals crackle on receive. Your transmitter output varies. VSWR is all over the place. Stations worked report you dropping out. It could be bad connections. Or connectors incompatible with one another. 

This video talks about the differences you may encounter between BNC sockets and the effects they can have on station performance. 

Have you had bad connector experiences? If so please leave your comments below. 

PS: Enjoy this blog? Wish to support it? Please store this link to Amazon Shopping as a bookmark or favourite. Then, when you buy something I receive a small commission (at no extra cost to you). 

Friday, May 17, 2019

Broadband RF noise detector

OK, so it's not an antenna or an accessory but it can still be handy around the shack. Wave it near a modem or switch mode power supply to see how much RF it chucks out. Hear aircraft at an airport or the local AM stations (all at once!). 

I'm talking about a broadband AM receiver. It's very simple. Basically a crystal set followed by a high gain audio amplifier. See the circuit and hear a demonstration in the video below. 

I also suggest following the links and reading the viewer comments.

PS: Enjoy the Daily Antenna and my videos? Wish to support them? If so please store this link to Amazon Shopping as a bookmark or favourite. Then when you buy something I receive a small commission (at no extra cost to you). Or browse my books page to see if any titles appeal. 

Thursday, May 16, 2019

Loading up a goal post (or flag pole) as an antenna

Yesterday I posted about tree antennas. Some of the references used a Rowland ring to couple RF into the tree. What happens if you use something a bit more conductive than a tree? Would it still work as an antenna? 

I was intrigued so I tried it. I used a goal post on an oval near me. The video below shows my results. The post would only have been about 6 or 7 metres high. So not even a full quarter wavelength on 7 MHz. I don't think you'd call it a good antenna but I still made contacts. 

There are opportunities for other experiments. Eg what if you used both goal posts and two Rowland rings. Then fed it with coaxial cable. Would they act as phased verticals on receive and be directive? Or what about something bigger Eg the very tall light poles around the oval. Maybe coaxial cable (use the outer braid) wrapped around a flexible sponge cylinder would work? If you've tried this or similar ideas let us know your experiences in the comments below. 

PS: For more portable antennas, including ones that perform better than above, check out Hand Carried QRP Antennas. Favourably reviewed. Thousands sold. Available as low-cost ebook or paperback. 

Wednesday, May 15, 2019

Tree antennas

I've never tried it, though it's on my 'to do' list. And it's much easier to get rough comparisons in a short time with modes like WSPR. I'm talking about tree antennas. 

People have been thinking about tree antennas since the early days of radio. But their most famous use was probably in the Vietnam war. Also a few enthusiastic hams have been experimenting with them since. 

Here's some links about this growing aspect of antennas to enjoy: 

Using trees for radio transmission (the toroid-like coupling featured is called a Rowland Ring)

* George Squier Tree Antennas

If you've had experience with tree antennas (good or bad), please leave a note below in the comments. 

PS: Want even more practical antenna ideas? Consider this selection of antenna books. They are affiliate links meaning that I receive a small commission (at no extra cost to you) if you decide to purchase.


Tuesday, May 14, 2019

Big wheel antennas for VHF/UHF horizontal polarisation with gain

VHF / UHF SSB operators normally use horizontal polarisation. Beams are easier to erect as there is no need to use a non-metallic mast. And there are some other advantages of being horizontal, for instance it being better for tropospheric propagation.

The main disadvantage of being horizontal is that almost all antennas are directional.  That's a problem if you're mobile and you're going around corners. Or you live in an apartment building and don't wish to put up a large beam and rotator.  One possibility is the halo that we discussed previously. However it has little gain. 

Another possibility is the 'Big Wheel'. It's still omnidirectional (in the horizontal plane) but it has a bit of gain. It comprises three full wave loops in parallel bent around a bit. Think of a Mercedes logo high in the sky. They draw attention if used on a vehicle but, believe it or not, ardent mobile VHF SSBers use them. And they're not too conspicuous if at home provided you can mount them high enough. If you're really keen you can stack two for more gain. Even if you already have a beam for VHF a Big Wheel can be handy to  have as an omnidirectional antenna for casual tuning around. Then when you find a weak signal, possibly due to a bit of lift, you switch to the beam and hear it better. 

Here are some Big Wheel articles and ideas: 

PS: Want even more practical antenna ideas? Consider this selection of antenna books. They are affiliate links meaning that I receive a small commission (at no extra cost to you) if you decide to purchase.


Monday, May 13, 2019

Should you spend more on antennas or transceivers?

Another video today. This time on a question a lot of beginning hams ask. Should you spend more on antennas or transceivers. There's not a simple answer but I hope that what's in the video below helps.

PS: Are you, or someone you know, returning to amateur radio? If so Getting back into Amateur Radio can help. Available as an ebook for $US 5 or in paperback it's full of tips to get you back on track.

Sunday, May 12, 2019

100 watt magnetic loop for units and apartments

Just about my most popular video ever, here's an antenna idea if you're very space-confined and wish to get on HF. If possible operate it from a balcony, courtyard or spare room rather than the shack to minimise RF exposure.

PS: Want to know more about other facets of amateur radio? If so 99 things you can do with Amateur Radio can help. Favourably reviewed, it's available in both ebook and paperback form. $US 5 for the ebook edition.

Saturday, May 11, 2019

More gain with an extended double zepp

We all know about the half wavelength dipole. A quarter wavelength of wire either side of a low impedance (normally coaxial) feedline. It's easy to build and a good performer in its resonant band. 

What happens if you lengthen its elements by say 20 or 30 per cent? The first thing that springs to mind is that the VSWR shoots up. The load presented to the feedline at your favoured frequency is no longer purely resistive. Your transceiver may have difficulty delivering full power into this unfavourable load. And the coaxial feedline will contribute additional loss. It doesn't sound very good does it? 

What if keep lengthening so that the antenna is twice as long. That is half a wavelength per side? It's now, once again, a resonant length. So there won't be much reactive component. But the impedance will be incredibly high - in the thousands of ohms. That's a big ratio when compared to the 50 to 75 ohm normal for coaxial cable or the transceiver's 50 ohm. So again the antenna is unusable. 

If you keep that feed system you have to go all the way to 3/4 wavelength per side for the impedance to drop enough to again work effectively with coaxial cable. That is a length three times it was compared to the original half wavelength dipole. That can work, as people who use 7 MHz dipoles on 21 MHz can attest. 

Still, one shouldn't write off intermediate lengths. If you change the feed system so that it's 300 to 600 ohm open wire feedline then the ratio between your thousands of ohms and the feedline isn't so bad. Any additional loss is very minor. And you can keep the transceiver happy with a good quality balanced antenna coupler between it and the open wire feedline. While there's extra adjustments it allows you to use certain lengths of antenna element that you couldn't before. 

One such length is 1.25 wavelength end to end. That's 0.625 wavelength per side. Or, 5/8 in fraction form. That fraction will be familiar to users of VHF vertical mobile antennas. For it is the length that gives optimum low angle radiation, that is radiation directed straight towards the horizon. For such purposes 5/8 wavelength long verticals work better than 1/4 wavelength verticals. 

With the antenna horizontal the main difference between a standard dipole that's 0.5 wavelength end to end and one that's 1.25 wavelength end to end is its gain. Both antennas are bidirectional. But the longer antenna's lobes are longer and narrower. In other words more gain. If you've got lots of space between antenna supports and wish to concentrate your signal then the longer antenna may be the better choice.  

Such an antenna is often called the extended double zepp. Or double extended zepp. It's a low cost  bidrectional antenna with a little gain above a regular half wavelength dipole. The diagram above shows an unspecified length of feedline to an antenna coupler. That can allow it to work on multiple bands. But if you're only interested in one band you could use a certain length as a matching section and connect coaxial cable via a balun.  

It's worth noting that many people have extended double zepps without them knowing. For example you sometimes see tuned feeder dipoles that are 25 metres (or 88 feet) end to end. That makes them a bit shorter than the G5RV (and inferior to them on 80m). But on 20 metres (14 MHz) the 25 metres is 1.25 wavelength, forming an extended double zepp there. Similarly the half size 12.5m (44 ft) version would be an extended double zepp for 10m metres (28 MHz).  

Want to know more about extended double zepps? Some good articles are here: 

OK, so we've talked about the extended double zepp? What's a double zepp, you might ask? That's a little shorter, only 1 wavelength from end to end. That's the high impedance example mentioned earlier. It has a little less gain than the extended double zepp.  

PS: Want even more practical antenna ideas? Consider this selection of antenna books. They are affiliate links meaning that I receive a small commission (at no extra cost to you) if you decide to purchase.


Friday, May 10, 2019

The ZL Special directional array

Yesterday we discussed the W8JK bidirectional array.

I suggest reading that now if you haven't done so.

The gist of the W8JK is that it's a two element bidirectional antenna. Both elements are spaced about 1/8 wavelength apart. Both are driven, with a 180 degree phase shift between the two. This is done by reversing the polarity to one of the elements.

What happens if you try other phase shifts? The answer is that the antenna becomes unidirectional like a yagi. Although they won't have any more forward gain than a 2 element yagi, there may have a deeper null off the back. That can reduce interference pick-up. Maximum front to back ratio happens when there's a 135 degree phase difference between elements. That configuration is sometimes called a 'ZL Special'. While the most common arrangement has two elements, people have added more for higher gain, particularly on VHF.  

Intrigued? Read more about ZL Specials here: 

* The 'BRD Zapper - A quick and easy ZL Special
The '8JK revisited and the 'BRD zapper
* Jonathan Hare's ZL special for the 2m band
* VK2ZIW ZL Special for 6m
* W4RNL on the ZL Special & more here
* JA6HIC's ZL Specials
* W7EL's FD Special

PS: Do you sometimes come across terms that you're unfamiliar with? The Illustrated International Ham Radio Dictionary can help. Available in both ebook and paperback it's great value. Find out more here

Thursday, May 9, 2019

The W8JK bidirectional array

Want a little antenna gain but in both directions? For instance if you want to cover up and down a straight coast. Or maybe long and short paths simultaneously for DX.  If these apply a W8JK may be worth building.  

Very much an experimenters antenna, the W8JK dates from the 1930s. It's named after the inventor's callsign. I don't know of any commercial versions of it so if you want one you'll have to make it yourself. You will also need to be willing to use open wire feedline. However in return you can load it up on multiple bands. 

What is a W8JK? It's basically two centre fed dipole elements spaced closer than what you'd normally find in a 2 element yagi. 1/8 wavelength spacing is typical. Unlike a yagi both are the same length. And both are driven with the feedline connected to them. Although, as you can see in the twist above, the connections to one element are reversed. That provides a 180 degree phase shift. If you've ever looked closely at a log periodic TV antenna you will see the same technique used there. 

The connections between the elements to the feedpoint are typically made from (now scarce) open wire TV feedline. Or you can use ladder line such as sold by the better amateur radio outlets. More ladder line extends from the feedline to the shack. A balanced antenna coupler is used to allow the antenna to operate over a wide frequency range. Because this looks after impedance mismatches, no trimming of the antenna is required.

As mentioned before, the W8JK is bidirectional broadside to the elements. There is a significant null off the sides that can help reduce interference pick-up. And if you move its feedpoint along towards one of the elements you can obtain some directivity. More on that tomorrow. In the meantime, enjoy these articles. 

* W8JK by ZR6TXA
* K1OJ W8JK wire antennas
* K5LJ multiband beam based on W8JK
* AF6SA modified W8JK beam
* F5RDS' disappointments with the W8JK

Have you had experience with W8JKs? If so please let us know in the comments below.

PS: Want to get more from low power amateur radio? Or even do better with your 100 watts? If so Minimum QRP can help. Available as paperback or ebook thousands have been sold. Find out more here

Wednesday, May 8, 2019

The best rope to support wire antennas

You might be able to get away with something cheap for a temporary portable antenna, but for something more permanent then the type of rope you suitable for portable antennas becomes important. Cheap rope frays, rots, disintegrates in the sun and snaps (personal experience speaking!). It's worth paying a bit more for good rope if it means that your antenna stays up for longer. 

The Radioworks site has a summary of the characteristics of synthetic rope materials. In summary, polyester (Dacron) and Kevlar (with a jacket) are good.

Here's some more thoughts on support ropes for antennas:

* 5 page QRZ forum thread
* A similar thread on eHam
* Discussion hosted by K5ZD

What antenna support rope do you use? What's succeeded and what's failed? Your comments are invited below.

PS: The items below may assist your experiments.  They are affiliate links meaning that I receive a small commission (at no extra cost to you) if you decide to purchase.


Tuesday, May 7, 2019

Antennas for 2200 metres (136 kHz)

Even more than 630 metre antennas, those for 2200 metres would appear almost impossible to accommodate in a typical suburban backyard. Even a horizontal half wavelength dipole would be over 1km long. And its effectiveness would be limited by its height of only a tiny fraction of a wavelength. And even the world's tallest manmade structure (Burj Khalifa in Dubai)  would be barely 1/3 wavelength high on 136 kHz.

Still, amateurs have had success with 136 kHz operating. Even from suburban locations. Well before 2200m became an amateur band, Americans have had access to a 160 - 190 kHz segment (often known as LowFER - or Low Frequency Experimental Radio). Antennas for this segment can be modified to operate on 136 kHz.

Here are a few LF antenna ideas to try: 

* N1DAY 630m/2200m antenna
* VK5FQ 2200m antenna
* K0LR computer modelling LowFER antennas
* ON7YD antennas for 136 kHz

 PS: Want to read more about antennas? Consider this selection of antenna books. They are affiliate links meaning that I receive a small commission (at no extra cost to you) if you decide to purchase.


Monday, May 6, 2019

The off centre fed (OCF) dipole

Most amateurs know about the half wavelength dipole. Two wires, each a quarter wavelength long, connected to a coaxial cable feedline in the middle. It's an efficient no-fuss antenna that provides good performance on a single band.  Variations include bending the wire elements down to form an inverted-vee (so you only need a single high support), adding a 1:1 balun or adding open wire feedline to allow operation on multiple bands. 

What happens when you move the feedpoint away from the centre towards one end? The answer is that the impedance increases. When you take it to its extreme you get an end-fed half wavelength wire with impedance in the thousands of ohm. At an intermediate point, about 1/3 the way along the wire, or 1/6 wavelength from the centre, you might get an impedance of 200 or 300 ohm. If you want to retain coax feed then you'll need to transform the impedance presented. That is typically done with a balun. 4:1 or 6:1 baluns are quite commonly used for this purpose. 

An attractive characteristic of this arrangement is that not only is the antenna suitable on the frequency that it's a half wavelength for but also even multiples. For example a 3.5 MHz OCF can work on 7 and 14 MHz. That's handy if you want to cover several popular bands from just the one wire.  

Want to learn more? Here is some more reading on OCF dipoles:

* DJ0IP OCF dipole
* K8JHR OCF dipole (pdf)
* W8JI on OCF dipoles
* VK2DQ OCF dipole (pdf)

Note that you will sometimes see reference to the 'Windom'. From a distance it looks the same as an OCF dipole. However it uses only a single wire feedline. That was suitable for the transmitters of the time where the wire could be tapped onto the output coil. Windoms now are rarely if ever used and what some people call a windom is likely to be an OCF dipole.

PS: Want to read more about antennas and other amateur radio topics? These books could be of interest. Available in both electronic and paperback.

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