On Thu, 11 Oct 2001 "Mike" <W4EF@dellroy.com> writes:
>
> For convenience, I match my coax fed 80 meter dipole
> in the shack with a tuner on 160 - 10 meters. On the higher bands
> its a real dog, even with LMR-400 feedline. I haven't made
measurements,
> but I am sure the line VSWR is very high in most cases. One of these
> days, I will get a better antenna :)
>
> 73 de Mike, W4EF..........
>
The feed impedance of an 80M dipole on 40, 20, 15, and 10M
will be quite high (thousands of ohms) because each side is
a multiple of 1/2 WL on those bands and thus are Voltage fed.
There is considerable mismatch SWR and considerable
additional coax loss due to this large mismatch.
Best to feed an 80M dipole with ladderline for ALL band use.
On 30, 17, and 12M, an 80M dipole operates as a current fed
Long Wire with a fairly low feed impedance (around 100 ohms
or so). Mine has less than 3:1 SWR on 30M (not exactly 3/2 WL)
and less than 2:1 on 17M (5/2 WL) and 12M (7/2 WL).
It is a KILLER antenna in the 4 main lobes that are centered
roughly 30 degrees from the wire. If the ends are dropped as in
inverted vee configurations, gain is reduced in the main lobes
and the nulls get filled in some.
One of my 80M dipoles is between two 130 ft towers broadside
to Europe. This antenna has often been the only antenna that
I could hear SE Asians on 12M during low marginal sunspot
levels. I have worked over 320 countries on each of the WARC
bands using various dipole antennas, including 3 80M dipoles,
a 30M rotary dipole (Matched boom of a TH6), and a ladderline
fed 20M dipole (figure 8 pattern from 10 thru 30M) at 50 ft.
Tom N4KG
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