Here are five ways you can test your antenna's effectiveness on transmit.
1. THE RANGE CHECK. The first way is probably what you did if you ever experimented with small transmitters like FM wireless microphones or toy walkie talkies. You'd have a transmitter going at home (keyed up by a friend or family member) and then go for a walk around the block to see how far it went. If you were lucky it might go say 10 or 20 houses. When you were a bit older and had something bigger then you might do the same thing in a car with a mobile setup. Ideally you'd be driving in all directions to get an idea of when your signal fades almost to noise level and becomes unreadable. On VHF or UHF FM you could put together a range tester (some don't need soldering) then go for a walk or drive.
2. REPORTS FROM STATIONS. Here is where you get on air, make contacts and ask for reports from multiple stations. A single contact at a single time is not enough - conditions can vary so much that comparisons can be misleading. And S-meter readings on different transceivers are meaningless. Different transceivers indicate different things on the same signal and people may or may not have their RF preamp or attenuator switched in. You also need to get reports from stations over a range of distances and directions. That is because some antennas might be outstanding for DX but mediocre for closer in due to the angle of their main radiation lobe. Results by direction can also vary.
The best sort of test is an A/B comparison. That is you have both antennas up. During a transmission you use an antenna switch (like we discussed on 26 February) to switch quickly between the antennas while asking for reports. Preferably done on a group or net where there's multiple stations on. Because many hams are biased with what antennas 'should' and 'shouldn't' work well it's best not to disclose what antenna A and antenna B are until after the test.
Another test, if you can't have both antennas up at once, is to be active at a certain time each day to try to work the same group of stations. Both before and after you erect the new antenna. You might want to drop power so your signal becomes more marginal and any differences between what your antennas can do are magnified.
3. ONLINE SDRs. Not much activity around? Here's where you do your own tests. Find an online web SDR and listen to your own transmission, switching between A and B antennas as you do so. Try ones in as many places and on as many bands as possible. Then repeat at different times on different days.
4. DIGITAL MODES. A great thing about digital modes like FT8 and WSPR is they give a dB signal to noise ratio of your signal. And in the case of WSPR you can leave your station to do its thing, coming back later to check results on WSPRnet. Look over results and average them to compare antennas. Or you could try to standardise for conditions by checking your performance relative to other stations near you. There are also more advanced WSPR-based antenna test tools like DXplorer.
5. CONTESTS. Contests are competitive. In the big ones there are multiple high power stations on the air. The signal reports won't be any value (they're all 5/9) but the fact that numerous stations in a particular part of the world can hear you well enough to get your callsign is. The big contests bring out people from all over the world so you can get an idea of your antenna's favoured directions.
Antenna performance on receive is another ballgame of fish. It may or may not be related to transmit performance. This is because on receive our aim is best signal to noise ratio rather than absolute maximum strength.
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.
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