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.


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