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11-12 of 12 messages
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Page 2 of 2
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RE: Fan Dipole and SWR measurement
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by WB6BYU on November 17, 2008
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Mail this to a friend!
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You can't count on "standard measurements" to get such a dipole exactly
on resonance - there are too many variables. Most of the formulas
you see published are simply estimates. In some cases someone may
have built an antenna and measured it, but there is no guarantee that
your environment will be the same as his. Minor change like the
spacing between the wires, the wire diameter, type of insulation, how
the ends are tied, whether it uses a balun, height above ground, or the
presence of another antenna 50' away can all change the resonant
frequency of a dipole.
Here is how to check the antenna with your SWR analyzer: start with
the lowest band you are using (in this case, 80m.) Tune the analyzer
through the band and find the frequency where the SWR is lowest. It
should be somewhere close to the band, perhaps 4.3 or 3.4 MHz. From
that frequency you can estimate how much you have to shorten or
lengthen your antenna to move the minimum SWR point down to where
you want it in the ham band. The 80m tuning will affect the 40m tuning
more than the other way around, which is why you adjust it first.
If you don't find a resonance point near the desired band, suspect an
element is not connected properly or a short or open in the coax. Note
that an SWR of 100 : 1 at the antenna will give an SWR in the shack of
25 : 1 if the coax losses are more than 1/4 dB, so (presuming the
analyzer is correct, which may be a stretch at that load) it seems likely
that you have a bad connection somewhere rather than a mis-tuned
antenna.
Getting a good connection to the coax shield is often a problem - check
your coax for bad solder joints, etc.
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RE: Fan Dipole and SWR measurement
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Reply
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by KE4SWA on November 19, 2008
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Mail this to a friend!
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I built an antenna like you described and it works fine. The problem I see from your post is that your lengths are wrong. They should be as follows: 3.6MHz the total length should be 124.8ft or 62.4ft per side, for 3.9MHz 115.2ft or 57.6ft per side,for 7.2MHz 65ft or 32.5ft per side. These lengths will get you a reasonable match with 75 ohm coax but would be better with 50 ohm. If you do not have 50 ohm on hand then 75 ohm will work with a little tuning, however the best match you will get is around 1.5:1 or there about.
Good luck, it will work once you get a good match.
73's
Phil
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