[MarinTeams] Advice from an old engineer regarding HF wire dipole antennas

[Bill]

>From the nanovnav2 at groups.io newsgroup:

Here's some advice from an old antenna engineer- you may already know all
this but I hope it may prove useful.

I assume you are discussing a half-wave dipole fed at the middle.  As has
been discussed, it is difficult to make a good measurement of an end-fed
quarter wave because you don't know what the VNA sees as the counterpoise
for the wire.  I suggest you not worry about the dipole balance, and I
assume you cut the two halves to be about the same. If not, read on.

The impedance match of any antenna is not very important if it is below 2:1
SWR, because the power lost is about 1/2 dB, which is virtually undetectable
in communications.  My only exception to this is that some modern
transceivers want to see 1.5:1 SWR for full power out.  If you are getting
1.5 or less, you really don't need to do better.

If you need to do better, here's how the wire affects the impedance.  For
the half-wave dipole, the resistance will be around 50 to 100 ohms at the
center.  If you move the feedpoint off center, the resistance will increase
slowly until you get near the ends, and at the end you will have a
resistance of thousands of ohms.  So if your feedpoint is off by a few
percent, it will make no practical difference.

The reactance is largely determined by the overall length of the dipole, so
trimming either end to get the reactance to near zero will work, even if the
two halves are unequal.

An example of this is the off-center fed dipole, where the two halves are
unbalanced by 50% or so.  The resistance is higher, but the resonant length
is about the same.

We engineers often have to decide what is good enough.

73  Ed  W2EMN


To that, I can add the SWR of antenna is affected by height above ground if
the antenna is less than ¼ wavelength in the air.  You can tune for 1:1 by
raising and lowering antenna height.  Best to get the 
antenna as high above ground as possible, and then tune for 1:1 SWR by
adjusting the length of the wire.  Just remember to raise the antenna to the
SAME height after adjustment.

73 de Bill, AB6MT