On 12 Oct 2006 23:32:20 -0700, "
wrote:

If I only have room for a short dipole, say 40 feet, and use a tuner such
as the SGC or Icom AH-4, what is the penalty in DB for the lower bands
(160, 75, 60, 40) compared to a half wave on each band?

Some quasi-wild guesses for a 40 foot dipole relative to full size at
the same height, probably best case.

160m: -16dB
75m: -8dB
60m: -3dB
40m: -1.5dB

These numbers don't mean much. I used EZNEC to look at the losses in
Q=100 loading coils with enough reactance to bring thing to resonance
on each band. No attempt to match to 50 ohms was made, but I made the
Q kind of low to reflect other tuner losses. Still, these are almost
certainly optimistic, especially on 160.

Dan, looking at a hypothetical unloaded scenario...

I don't know what feedpoint Z you got for a centre fed 40' dipole at
7MHz... but lets guess somewhere about 11+j750. Lets feed the antenna
with 15m (~50') of RG58C/U for a loss of 18dB (efficiency 1.6%) and an
input z at the ATU of 44-j179. The ATU should handle that with very
low loss... so overall, the efficiency is ~1.6%.

You can do these calcs with the line loss calculator at
http://www.vk1od.net/tl/tllc.php, nothing to unzip!

Adding 26' of wire to the dipole should improve the antenna to better
than 80% efficiency or a fifty fold improvement on transmit. But it
only worked so dramatically because the 40' dipole centre fed with
coax is so inefficient.

Owen
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