Monday, February 18, 2019

Big thick efficient HF magnetic loops for the lower HF bands

Magnetic loops are often thought of as being compromise antennas, used by apartment dwellers and others who lack the yard space for a full sized antenna. Many are fairly simple slapdash weekend projects, taking only a few hours to build. While contacts can certainly be made with such antennas, performance is often well below possible due to high resistive losses. 

These often arise from a combination of (i) inadequate loop length, (ii) poor choice and diameter of loop material, (iii) losses in the variable capacitor and (iv) high resistance connections, especially between the loop and the variable capacitor.  In addition compromised parts can mean that the loop handles only a fraction of the legal power limit.  Add the loop's losses with the reduced signal due to a restricted output power and suffer the consequences.  You may still do well on efficient digital modes like WSPR but you will likely be the weakest station in the group if on an SSB net.

It doesn't have to be like that. With time, money and effort you can build a heavier, more efficient loop that can take the legal limit.  Your signal will then be quite strong. In fact under certain circumstances you might do better than some with full-sized antennas.  That's even true on lower bands like 80 and even 160 metres.

Building a big thick and efficient HF magnetic loop is more than your average casual weekend project. But the results will be worth it. Here's a few resources from those who have succeeded in this quest.

VK4AMZ magnetic loop for 160, 80 & 40m
W7YV magnetic loop for 160, 80 & 40m
VK5SFA 160m magnetic loop design
VK5SFA's 160m magnetic loop videos
AI6TK 160 and 80m magnetic loop (YouTube video)

Even if you don't attempt one, these links should at least give you a renewed appreciation of what magnetic loops can do when attention is paid to minimising losses from all sources.

PS: I have written five books on amateur radio topics. They are available in electronic and paperback form (most countries). Ebooks are under $US 5 each. Find our more here or follow VK3YE Radio Books on Facebook .

Sunday, February 17, 2019

The VK2ABQ multiband mini beam

Before the Moxon Rectangle became popular there was the VK2ABQ beam.

Like the Moxon, this was basically a two element yagi with the ends bent in to reduce turning radius. As originally presented, in a 1973 Electronics Australia magazine, the VK2ABQ had wire elements for 10, 15 and 20 metres. Its square shape make it look like a rotary clothes hoist.  Elements were bent in towards themselves, with large coat buttons providing the spacing.

This development was done before amateur access to antenna modelling programs and antenna analysers. Test equipment generally available then would have been little more than noise bridges, RF ammeters, signal sources and field strength meters.

The VK2ABQ allowed people who didn't have the room or money for the standard triband trapped beam to have something of almost similar performance in a smaller space for less. The antenna became more widely known through Pat Hawker G3VA's Technical Topics column in RadCom. The critical coupling technique it used was refined by Les Moxon G6XN to produce what we now know as the Moxon.

VK2ABQ as built by ZS1JHG
Modeling and understanding small beams (including VK2ABQ) by W4RNL
Vertically polarised VK2ABQ for 50 MHz by GW0GHF
Critically coupled antennas by VK6APH/G3WXO

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.

Saturday, February 16, 2019

Resistive antenna bridges

You're listening to a weak signal. There's fading, there's static, there's a station a little too close for comfort. But you can just about make out all of their transmission.

That is until the carrier appeared. Your S-meter swings wildly between S9+20 and S9 +40 as adjustments were made. When you thought it was over it appears again.  Followed by speech.  "Haarlo Harlo 1 2 3". Apparently oblivious of or uncaring towards the activity on the frequency.

All that could have been prevented if the other station used a resistive antenna bridge. This is a small device that allows your antenna coupler to be adjusted without radiating a signal.  True, they don't measure VSWR or RF power output. Nor do they replace an antenna analyser. But they are cheap, self-powered, and, if you wish to build one yourself, requires fewer than ten or fifteen parts. They also protect the transceiver's final from wild impedance swings that can occur when adjusting the antenna coupler.

A resistive antenna bridge is just a few resistors, capacitors and a potentiometer. Oh, and some sort of indicator that shows the null you get when you've tuned up correctly. That could either be an (increasingly rare) meter movement or LED. Or, if a voltage controlled oscillator is added, an audible indication.

 Here's a few bridge ideas if you'd like to build your own.

GQRP Club resistive SWR bridge by G3ROO & G4WIF (PDF)
Determine SWR with a resistive divider Circuit Salad
ZS6AZP's investigation of a resistive bridge
G0WQR resistive bridge with LED from Sprat
L-match antenna coupler with resistive bridge from my site
Inside a resistive antenna bridge my video

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.

Friday, February 15, 2019

The Bi-Square

Another rarely heard antenna on the air is the bi-square. Probably due to its size. And many prefer coaxial cable to the 300 ohm ribbon feedline that the bi-square normally uses.

If you think a single element quad loop (1 wavelength perimeter) is large, double its size to get an idea of the bi-square's dimensions.  But despite what it looks like from afar, the bi-square is not quite a real loop since it is split at the top.

Still, if you have one tall tower (at least 3/4 wavelength high) the bi-square may be worth considering. It's basically a bidirectional gain antenna. If you've already got the wire the construction cost is low. And you may be able to use some ingenious remote switching with relays to change the antenna into a large loop for other bands. 

Bi-square ideas and discussion here:

The bi-square array reappears N4LBJ's 18 MHz version
Bi-square radiation How they work via HA5CFJ (Hungarian)
Curtain antennas W8JI explains how the bi-square is related to the Lazy H
Bi-square for 2 metres Manageable dimensions up here. By KG4ZNC
28 MHz bi-square used portable My one and only bi-square test (video)

PS: Into low power amateur radio? Covering equipment, antennas, operating and strategy, Minimum QRP is the book for you. Available in both ebook and paperback (some countries).

Thursday, February 14, 2019

Making your own open wire feedline

One of the most versatile multiband HF antennas is a doublet fed with open wire feed line. The antenna's length is not particularly critical provided it is longer than about 3/8 wavelength on the lowest operating frequency.  With a good balanced coupler this arrangement will provide low loss coverage of maybe 8 or 9 bands. 

Some find it difficult to find suitable open wire feedline. What you need must be able to withstand the weather and have a characteristic impedance between about 450 and 800 ohm (not critical).  Its wire thickness also needs to be suitable if you run high power.  You may be lucky and find some commercially at a reasonable price. Or you may not. And even if some could be ordered, you may have the urge to make an antenna that requires it today!

Fortunately it's fairly easy to make your own open wire feedline. Techniques vary.  Spacers can be made from a variety of non-conductive low RF loss materials. You may even be able to fabricate your own if  you have access to a 3D printer. Enjoy the links below for some ideas:

Feeding  your station QST article on various feedlines
Home made open wire feedline by W1AEX
Home brew light weight open wire line by N5ESE
Open wire Ladder Line Includes accounts of testing by DJ0IP
Some old notes on parallel transmission lines by W4RNL
Why use ladder line by KV5R
Making open wire antenna feedline One of my videos

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.

Wednesday, February 13, 2019

Pedestrian mobile HF magnetic loops

OK, I admit it, this is one of my hobbyhorses. Most of the links presented below are from my website. But it's a fun novelty aspect of amateur radio, not only for yourself but the people you work. And the construction of antennas light enough to carry for long periods makes you even more aware of the trade-offs between efficiency and weight.

A pedestrian mobile magnetic loop can occasionally give 10 000 km plus contacts. However up to 3000 km is much more common, especially at the current phase of the solar cycle. While I've tried coaxial cable and even mains flex for the radiating element, aluminium strip seems to provide the best trade-off between lightness and efficiency.

A bit like having different camera lenses for different jobs, I suggest having at least two loops.  a larger loop is best for the 40 - 10 metre range.  While a smaller loop is more convenient if you just want 10 and 6 metres.  Descriptions of these and other loops are in the links below:

Alexloop portable loops Loops to buy if you don't wish to build your own
Portable HF transmitting loop antenna  A 14 - 30 MHz collapsible PVC pipe loop by N5IZU
Portable HF magnetic loops Presentation Notes by WD6DBM (PDF)
Ultralight ped mobile mag loop for 40 - 10 metres When you want the very lightest
A better ped mobile mag loop for 40 - 10 metres A little heavier but a lot better
SummerLoop mag loop for 15 - 6 metres Just the thing for the summer sporadic E
Low cost magnetic loop for 10 - 12 metres YouTube video by RadioHamGuy

PS: Magnetic loops (and many other antennas) are covered in Hand-carried QRP Antennas. Available in electronic and paperback form (some countries) this well-reviewed book is a popular read amongst hams who go portable and pedestrian mobile.

 Link to find out more about Hand Carried QRP Antennas

Tuesday, February 12, 2019

Using salt and vinegar to clean corroded wire

No antennas today but you should still find the post useful.

It deals a problem we all have with outdoor wire antennas. Wire corrosion. 

This is a video demonstration I made of cleaning corroded wire.

PS: To support The Daily Antenna, please start your Amazon shopping here . You won't be charged extra and I'll get a small cut from any purchases you make (affiliate link). You can buy lots of stuff there, including electronic parts and my books.

Monday, February 11, 2019

The HF folded dipole

Today's antenna, the HF folded dipole, doesn't seem to be that widely used among amateurs.

Maybe because it needs twice as much wire. Instead of just one wire split in the middle at the feedpoint, folded dipoles have two closely spaced wires joined at the ends. One wire is split at the feedpoint while the other is continuous. On the other hand having all of the element at ground DC potential is sometimes seen as an advantage with users lauding its quietness as a receive antenna.

Folded dipoles have a 300 ohm impedance.  Unlike other types of dipoles and verticals this makes them unsuitable for directly feeding with 50 or 75 ohm coaxial cable without a 4:1 balun. On the other hand the higher impedance can be beneficial for long feed line runs as loss is lower. Why? High impedance means relatively high voltage and relatively low current. The voltage drop caused by a given line resistance is less significant at higher than lower voltages. For a similar reason this is why electricity grids operate at many thousands of volts over their long distance transmission lines before being stepped down to the normal domestic AC voltage.

If resistors are installed at certain places a folded dipole can be made to operate over a wide HF frequency span without an antenna coupler. Performance-oriented amateurs don't like this due to the extra losses introduced.  Commercial and military users, who primarily use HF for strong-signal single hop communication, often prefer versatility and usability over efficiency.  For them a broadband folded dipoles, such as a T2FD, may be more appealing than it would to a distance-oriented amateur DXer or efficiency-minded QRP enthusiast.

I haven't worked with folded dipoles so can't speak from practical experience. But these people have so their accounts are worth reading.

Folded dipole antennas Video by W1GV
Notes on the terminated wideband "folded dipole" Evaluation by W4RNL (PDF)
Terminated Tilted Folded Dipole by PA0FRI - Dutch
The folded dipole W8JI finds negligible benefits in using them

PS: I have written five books on amateur radio topics. They are available in electronic and paperback form (most countries). Ebooks are under $US 5 each. Find our more here or follow VK3YE Radio Books on Facebook .

Sunday, February 10, 2019

The single element HF wire vertical delta loop

Only got a single high support? Don't have quite enough room for a dipole? After a loop style of antenna with a reputation for low noise reception? Or want something that can easily be changed from vertical to horizontal polarisation?

If so then the single element delta loop could be your saviour. It's a simple antenna to build comprising a wire approximately one wavelength perimeter. Feed it at the bottom centre for horizontal polarisation or down the side for vertical.  If you've got two supports you can have it pointing to ground (with a higher average height) or if you only have one you can have the apex pointing up.  The latter is particularly convenient for portable operating where you can use a telescoping fishing pole mast to hold it up.  It doesn't matter very much if it's not quite an equilateral triangle.

A single element delta loop doesn't have a lot of gain. Possibly a shade over a half wave dipole. But in its vertically polarised version it can give good low angle radiation, particularly if near salt water.

A challenge with the delta loop is how to feed it.  That's because its impedance is quite a bit higher than 50 ohms. It's not quite like a ground plane (about 35 ohms, but higher if you droop the radials) or a half wave dipole (about 70 ohms, but lower if you make it an inverted-vee). Options include using a matching section (a quarter wavelength of 75 ohm coax multiplied by its velocity factor), some sort of transformer or open wire feed with a balanced antenna coupler.  Open wire feed will allow the loop to work on multiple bands down to about 70% of the design frequency. The links below will give some good ideas on feeding this antenna.

40 - 10 m delta loop Comprehensive article by GU3WHN (PDF)
Multiband mono delta loop by DU1ANV (PDF)
$10 No transmission line delta loop Eham article by KE7WAV
Vertical delta loop Elmer Hour beginner guide from the Villages Amateur Radio Club (PDF)
Amongst the Kilowatts Part 1 I describe and demonstrate a 14 MHz portable delta loop (video)
Amongst the Kilowatts Part 2 More on my 14 MHz portable delta loop (video)

Start reading, start watching then start building. They're so cheap that you could even make several.

PS: Looking for even more things you can do with amateur radio? 99 things you can do with Amateur Radio can help. Available in both electronic and paperback.

Saturday, February 9, 2019

The quarter wave ground plane

Those who got their start with 27 MHz CB will be familiar with the ground plane. It consists of a radiating element a quarter wavelength long connected to the coaxial feedline's inner.  Connecting to the braid are two, three or four radials a quarter wavelength long.  These may be horizontal, or if you want a better match to 50 ohm coaxial cable, droop down at about 45 degrees. Ground planes can be made either of wire (on the HF bands) or aluminium tubing (on VHF bands like 2 metres - 144 MHz).

The ground plane is a vertically polarised omni-directional antenna. This makes it good for communicating with mobile stations.  That includes working through repeaters such as available above 29 MHz.  On frequencies lower than that ground planes are preferred over low dipoles for long distance contacts due to their lower angle of radiation.  This is particularly so near the coast due to the beneficial effect on radio signals of large salt water bodies.

Ground planes are one of the best antennas a beginner amateur can build. Especially for bands like 10, 6 and 2 metres. Connecting one to a 2 metre handheld transceiver will greatly increase its communication range compared to the standard 'rubber duck'. And on 10 and 6 metres, even when there's no sunspots, a ground plane will help you work stations thousands of kilometres away over summer.

Check out these practical ground plane links:

Build a Portable Groundplane Antenna Improve your 144 MHz signal - ARRL PDF
Simple ground plane for 2m/1.25m/70cm From N1GY
20 metre three wire ground plane Practical article by KK5JY
10 metre ground plane antenna Video from KG0ZZ
Simple ground plane vertical Video tutorial on a 7 MHz ground plane by W1GV

Find a high support such as a fishing pole or tree branch. Then just get an SO239 socket and some wire to make your own ground plane hanging from your support. Even if radio conditions are poor and you're not hearing many signals, WSPR can give a clue as to propagation and how the ground plane is working relative to your others. 

PS: Puzzled about some of the terms you hear? This dictionary can help with over 1500 explained.

 Link to Ham Radio Dictionary

Friday, February 8, 2019

The coax-fed horizontal dipole

Just about the simplest antenna to make is the humble coax-fed half wavelength dipole (usually shortened to 'dipole').  As someone once said, it's hard to make an antenna that performs much better than one but easy to make one that's inferior.  It's a great choice if you're starting out in ham radio.

In its basic form it comprises two quarter wavelengths of wire connected to some 50 or 75 ohm coaxial cable. Take the band's wavelength in metres (eg 20 metres, 40 metres, 80 metres etc) and divided it by two. That's approximately the dipole's end-to-end length.  There are better formulas out there but you are still likely to need to trim it to your desired centre frequency during testing.

A high dipole (more than about 1/2 wavelength up) gives a low radiation angle, good for DX. A low dipole pushes more radiation at high angles.  While not so good for DX, high angles may be just what you need for close-in contacts on bands like 80 and 40 metres. Its radiation pattern is bidirectional. Like a figure 8 with maximum radiation broadside to the wire.

You can make a dipole from wire, or, especially for VHF and UHF, solid tubing. It can be erected horizontally or vertically, depending on the desired polarisation. And if you run out of room or only have one tall mast, it can droop down from the centre, forming an inverted-vee.  Or, with two non-metal end-masts it could droop down and even bend in at the ends to save space to form a shallow inverted-U.

Anyway enough from me, here's some great dipole links to peruse.

Antenna Here is a Dipole All the basics here in this great ARRL article (PDF)
Feeding Dipole Antennas Part 2 of the above from the ARRL (PDF)
Your first antenna - the half wave dipole A dipole starter from the RSGB
Optimising a coax fed half wave dipole Solid know-how from VK2OMD
Making and using a simple 40m dipole Popular David Casler YouTube video

Build a dipole from information in the above and you can't go wrong!

PS: Dipoles (and many other antennas) are covered in Hand-carried QRP Antennas. Available in electronic and paperback form (some countries) this well-reviewed book is a popular read amongst hams who go portable.

 Link to find out more about Hand Carried QRP Antennas

Big thick efficient HF magnetic loops for the lower HF bands

Magnetic loops are often thought of as being compromise antennas, used by apartment dwellers and others who lack the yard space for a full ...