I have a Cushcraft R7 that I started to play around with yesterday. It's
been through a few hands but is in good shape. I seem to remember
problems getting it to behave previously but put that down mostly to
proximity effects. Now I have a little more space I thought it might be
interesting to see how this thing does. Well I found that it still
behaves a little strange.
First thing I did was to measure the traps. Using a gdo and frequency
counter all the traps show apparent resonance a MHz or two below the
band they are designed for. Comparing this with a W3DZZ trap which shows
resonance at exactly it's stated frequency. Is this difference the
effect of the tubing at the ends of the R7 traps or are they
deliberately designed that way?
Assembling the antenna shows that 10m resonance and lowest SWR are at
about 28.5MHz using the manufacturers dimensions. All other bands
resonate and have minimum SWR below the design frequency, 24.7, 20.9,
17.8, etc. Additionally the bandwidth seems much narrower than
published.
Whilst I could shorten the bottom section that would place the 10m bit
too high up the band, but I cannot just shorten the bit above the 10m
trap, because it's one piece with the 12m trap. What to do?
Would adjusting the trap and antenna length to produce resonance at the
same frequency make the bandwidth wider or narrower?, and has anybody
any experience of adjust R7 style taps?
73's
Martin, G4UQF/HS0ZED
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