Trent Fleming wrote:
> In a recent ham magazine, there is an article about an HWV for 160m I am
> very interested in having an antenna for 160m, and in having a vertical for
> the low bands in general. If I were to build this HWV, are there any
> techniques I should consider to get it to perform well on 80 and even
> perhaps 40m? Or am I being greedy and just need to focus on the HWV
The concept of the helically wound vertical (HWV) is appealing: no
lossy loading coil, just a low loss wire. The "proof" of this is
that you can run a kW and the antenna does not get hot. It is easy
to see why this antenna is popular.
The reality is different.
The HWV does not eliminate the loading coil, it just replaces it
with a very long skinny coil that runs the length of the antenna.
The radiation resistance of the antenna is not affected by the
coil. However, the Q of a long skinny coil with wide spaced turns
is much less than a conventional coil. This is simply because it
takes a lot more wire to make the coil. In the article, over 250
feet of wire are used; much more wire than is required for a
conventional coil. If you want to prove it to yourself, just
do a few sample calculations using Wheeler's inductance formula,
which is in every radio handbook. You will see that if the length
of a long skinny coil is doubled, the amount of wire needed goes up
about 40%. The loss per foot of wire is basically independent of
spacing if it is at least a couple of wire diameters. Therefore, the
loss goes up 40% with each doubling of length. Also, winding the coil
on PVC pipe introduces additional losses due to the PVC.
Since the wire is distributed over the whole length of the antenna,
the great amount of additional heat is not apparent because it is spread
out so much. The radiation resistance is only a few ohms, but the wire
resistance may be many times the radiation resistance. This improves
the SWR (if not using a matching network at the base) and broadens the
bandwidth, but lowers the efficiency.
The antenna then appears to be satisfactory with a decent match and
decent bandwidth and able to handle high power. The low efficiency may
not be apparent since it is still possible to make QSO's. It may
compare favorably to a low dipole ("all" dipoles are low on 160 meters)
simply because, in many cases, low dipoles don't get out very well. It
may even be possible to work DXCC with an HWV. We know that doesn't
prove high efficiency because KH6DX has achieved 160m DXCC operating
mobile. His antenna is a very good mobile whip, but it is certainly
down 10 dB from an efficient antenna. Don makes up for low efficiency
with QRO and persistence. BTW, Don doesn't use an HWV.
It's not like an HWV is the only antenna you can put up on your city
lot. It's actually easier to just use an aluminum vertical with a
conventional coil. Why go to the trouble of making an HWV?
I see lots of coils going begging at every swap meet I go to. The
aluminum can be smaller diameter than the PVC, which may allow you to
get away with more height. PVC is a lousy structural material.
You will need a matching circuit, and the antenna bandwidth will be less
than an HWV.
Now as to the question of 80 and 40 meters, that will not work with
an HWV, but is easily done if a conventional loading coil is used.
Just change the tap to QSY. Some advanced systems use a motor
driven roller inductor to do this.
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