Antenna Summer – part 1

The first antenna I put up after we moved in here was a simple 2×5 meter dipole sloping down from the roof into our garden. I wrote about it – and the noise it received – here

Last year summer I finally had time to do some maintenance on my old CB-whip and put it up on the sheet metal roof.

I put it up mainly to get my station licence, because the amount of noise it received on shortwave was as much as the dipole, hence I didn’t log many QSOs with it. Still, it turned out to be a marvelous antenna to do medium- and longwave DXing and I logged some 285 different NDBs on longwave in the last 12 months.

But now, this summer, it is time to get serious about putting up a decent antenna. Being a sensible guy I set myself some goals:

  • The antenna has to be cut for the 20 meter band
  • It has to have lower noise pickup than the rooftop vertical
  • I have to use whatever materials I have lying around
  • Money can only be spend on the odd nuts and bolts

Then there is the question on how to determine what is good, better and best. My thoughts were to use WSPR to monitor how well an antenna would receive. If you do this scientifically you use two identical setups with only one variable: the antenna. Unfortunately I don’t have two IC-7200s, so I decided to go for an alternate approach: day one the first antenna, the other day the other antenna. This adds the variable of different conditions during different days, but over the course of a week or two you will get a broad picture, I reckoned.

The first two antennas I compared were the rooftop vertical and the sloping dipole. They were both resonant on 20 meters and I used that bands WSPR frequency to compare. After comparing for two weeks I found only a minimal difference: they were equally good/bad. I did get consistent beacons from VY0ERC, though (probably the most northerly located amateur radio club in the world). But since there are only a limited number of WSPR stations on 20 meters I decided to switch to monitoring JT65 stations for the next comparison.

I had another CB whip laying around and after gutting it and adding two 5 meter radials I put it up in the far end of the garden. Now our garden is only 10 meters deep, but at least it is at some distance from any noise source in our house and the neighbours.

This time I only tested for 10 days, but again I didn’t notice much difference. Even though the rooftop vertical was noisier it would still receive all the JT65/JT9 stations the garden vertical could receive. It was only that I wandered up into the SSB portion of the 20 meter band that I noticed a difference. Comparing stations from Hong Kong, Japan and central Russia was like comparing night and day. Night being signals barely coming out of the noise, day being comfortable copy. The final confirmation came from my Olivia friend Ken in Japan. He calculated an +18 dB advantage of the garden vertical over the rooftop vertical during our most recent QSO.

So part 1 of our Antenna Summer has been a success in the fact that I now know that the garden is a better place for an antenna than the roof top. Less noise, stronger signals. But the question now arises: isn’t a metal roof a very good counterpoise and shouldn’t it be beneficial to the workings of antennas? Why does it degrade the performance of my rooftop vertical?

Hans "Fong" van den Boogert, BX2ABT, is a regular contributor to AmateurRadio.com and writes from Taiwan. Contact him at hans.bx2abt@msa.hinet.net.

3 Responses to “Antenna Summer – part 1”

  • ve7sl Steve:

    Hans – this is just the opposite of what I would have expected as well. Perhaps that large metallic device on your rooftop (not sure what it is) is too close and degrading performance? Have you considered a wire loop with tuned feeders?

    Steve

  • ve7sl Steve:

    Ah…I read your original post again and see that it is your solar water heater 🙂

  • Boots VK3DZ:

    I recall the RSGB VHF Manual advised mounting a VHF aerial at least 12 m high to get away from short-range noise (QRN) arising from dwellings – perhaps moving your aerial into the yard has achieved the same goal?

    Since the RSGB advice (c.1980) we have been swamped in switch-mode PSU & noisy lamps so any separation between those & radio antennas would be helpful.

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