|PA8W Amateur Radio|
Small but effective antennas for the higher HF bands
search of the best small (but effective!) 20m antenna I tried quite a few
antenna options using a 12m telescopic glass fibre push-up pole. (from
This pole is very convenient to put up an antenna at heights that exceed the size of your lot.
I made some fixing points to my garage, starting off on the flat roof of the attached carport.
The attic of my garage is my shack, and through a little door I can step out of my shack directly onto the carport roof.
So, here I had a quick and easy platform for my antenna experiments.
Additionally, I always use a comparison antenna, since there is no way of properly judging an antenna without having a reference antenna in place. Quickly switching between the two antenna types gives you reliable data about antenna performance on stations at various distances. However, make sure that the two antennas have the same favoured direction, so your judgement is not blurred by directional properties of one of your antennas.
Over a period of 1,5 years I built and tested quite some antenna types.
The choices I made were inspired by the ARRL antenna book, by the great articles of W4RNL Mr Cebik, and last but not least my own humble opinion. (which I had to adjust a few times...)
Looking back I would have been better off simulating and modeling on the computer first, using MMANA or another nice program. But then, modeling doesn't take in account everything. Reception quality for example is highly depending on where your QRM is originating from. Is it local, seemingly originating from the horizon? Is it short distance but ionospheric?
We will not go into that science here but we will just note the differences we meet in our tests.
And another misunderstanding blurred my opinion about several antennas: Lots of antennas are specified or calculated in free space, which of course is not the real world. At least on the higher bands you need a pretty low radiation angle. I now judge 40m antennas at a take-off angle of 30 and 15 degrees, and a 20m antenna at 10 and 5 degrees, modelled at the correct height over ground. This way, the outcome of my antenna experiments show a pretty close match to the calculated data.
I stopped trying to get an easy 50 ohm match; the antenna performance is much more important, and a low loss match is almost always possible using a symmetric feedline and tuner.
I've put some data in a table, for easy comparison:
The numbers in the table show the gain in dBi at the specified take-off angle.
The blue row is the currently used 20m loop, top wire at 15 meter height.
For me, this row is to be used as a reference, since I look for improvement over my existing antenna.
Note that on 28.4MHz (as on 51MHz) the top wire is opened, so at this frequency this antenna
performs as a Bi-Square, and is less than 2,5dB down compared to a full size 2-el Yagi!
Also note that even on 40m, this is a very reasonable antenna, less than 2dB down compared to a full size 40m loop. This small difference is almost impossible to notice or prove in real life.
For DX (low take-off angles) height is much more important than size.
And this small and light loop can easily be pushed up a few extra meters, and still be rotated by hand if necessary.
The above antennas are all modelled, and some are built and tested.
Top element opened
Top element opened
As my experiments and simulations show, the performance of antennas may not entirely meet expectations or computersimulations in all cases.
Especially for a small lot, it is impossible to include all parameters of the antenna environment into the equasion, since the house plumbing and wiring, steel rebar, etc. will all couple to the antenna to some degree, and therefore distort the outcome. And there's also the local QRM environment; for example, verticals in my location show dramatic noise problems on reception. So, antenna modelling is great, but the results still have to be tested in real life.
Right now, the bottom fed loop is my antenna of choice for 20m and up.
This antenna has a small profile, small turning radius of 2.1m, and can be built very lightweight, since only one light spreader is necessary in the top, and the rest of the antenna stays in place using an even lighter bottom spreader, a few cable ties and gravity...
This also means that I can easily give it a lot extra height compared to a beam (which I can't put up here anyway) using much lighter hardware.
The performance of the loop seems to be even better than expected; in real life measurements it greaty outperforms any vertical I tried, much better than predicted by modelling.
So right now I am looking for a good method to automatically open the top of this loop on 10m, so I don't have to do that manually. The most promising method seems to be a 10m trap, but modelling shows it may cost about 2 dB on the 15m band, which is not desirable of course. For all other bands loss seems to be less than 0,5dB which is acceptable.
What I will do is build the trap concept, and bypass it with a relay, so I can measure exactly if real live loss is in accordance with the modelling data.