Somehow I missed the ON5ZO post so replying to K2AV's reply:
I am using just such an antenna. I have a full size 80 ground plane
currently with 18 quarter wave 80M radials attached 15 ft from ground.
Wanting to use this antenna on 160 I looked at solutions in EZNEC. It
happens that if I added a pair of horizontal or sloping wires near the
top of the GP I could reduce the high Z 160M feed point to something my
transmitters could live with. It also turned out one wire would disrupt
the circular nature of the antenna pattern, but two would not if the two
could be relatively short.
So I added a pair of sloping wires whose length was roughly decided by
EZNEC with a little tuning. I won't quote exact length here because each
installation will be different but mine are on the order 25 ft and slope
at 35 or so degrees. The wires are attached about 20 feet down from the
top of the GP, this height chosen because that is where tower stops and
aluminum pipe top section begins, so it was easy to attach there.
The two wires are fed through traps tuned to 80M so that they are
isolated on 80M but connected on 160M. I made the traps from toroidal
coils resonated with about 30 pF RG213 (about one foot of coax). I had
in mind relays but N6ND suggested traps which saved me lots of hardware,
thanks Rick!
This antenna is extremely effective on both bands. On 160 the match can
be varied with the horizontal wires. I chose horizontal wire lengths
that the active antenna coupler can live with so as to minimize pattern
distortion while delivering the power.
73 Art
Guy Olinger K2AV wrote:
>> "Franki ON5ZO" <on5zo@telenet.be> wrote:
>>
>>> Gentlemen, please your advice.
>>>
>>> If I were to convert my working resonant 80m vertical
>>> into a 80/160m
>>> antenna, I assume I have two basic options according to
>>> the literature:
>>>
>
> There are many options. They are reduced by embraced assumptions or a
> requirement of only direct 50 ohm coax feed.
>
> If one makes the concession that there may be a miscellaneous Z to match on
> either or both bands, and is willing to construct a matching device, quite a
> number of possibilities emerge. While many have the space and mechanical
> circumstances to erect one of several textbook antennas in current favor,
> IMHO most folks have constraining mechanical circumstances that prevent any
> easy use of the currently favored designs.
>
> I saw one installation that had a 80m vertical wire which was supported by
> an insulator well off-center on a rope between two decently tall trees.
> The feed point directly below was dictated by an out-building and the wire
> rose nearly straight up. The radial mix was necessarily eclectic
> and shortish, dictated by fences, house, patio, alley way, etc. The problem
> was to add 160 to the only possible low band wire with any height on the
> property.
>
> My suggestion was to convert the portions of the rope clear of the trees to
> wire. One side being well longer than the other, it was not "pure" and
> (gasp) would have some degree of horizontal polarized radiation. It also
> would present non-standard feed Z's on both bands and require matching.
>
> Once the commitment was made to match whatever impedance presented at the
> base on 160 and 80 and switch matching devices, the remaining problem was to
> measure the eclectic Z on both band and build a match. The necessary
> match components were read off the menu of a K2, having satisfactorily
> matched both Z's with its auto-tuner. A pair of iron-powder toroids wound to
> value, a pair of transmitting caps, and a couple of relays in a weatherproof
> box did it.
>
> The presentation of match components after autotune can now be done with a
> K3 as well.
>
> A miscellaneous wire solution that is actually possible at a given site can
> be modeled in advance to avoid extreme Z's for match, and if series ground
> loss can be minimized, results will be good. A good design on 160 consists
> of two considerations with everything else in 12th or 13th place: the least
> resistive ground/counterpoise possible and enough effective length to raise
> the feed point impedance to minimize whatever practically unavoidable loss
> remains in the ground/counterpoise system.
>
> The end result was that he had a passable (R component of feed Z greater
> than series ground resistance) 160 antenna that got him on top band, without
> being severely narrow, an 80 meter antenna that clearly worked for DX *and*
> USA, and was less visible than before.
>
> Purity of design is highly over-rated on 80 and 160 and does not trump being
> on both bands.
>
> Stations on the receiving end cannot tell that an antenna is or is not all
> vertical. Miscellaneous polarization doesn't show up pink rather than green
> on anyone's receiver.
>
> 73, Guy.
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