It is going to have a pretty high impedance, just how high is hard to
predict but the 50ohm swr is likely higher than 10. So an "open wire
feeder" and a matching section is called for.
For something that is an maybe scalable approximation, I just put up a
DX Eng 80m dipole kit with 300 ohm feeder and use it on 80/40/20/17/15
with an LDG Pro II tuner (it tunes everywhere in these bands). You need
to have a tuner. I cut the 300 ohm feedline per DXE's recommended odd
multiple of lowest frequency one eigth wave i.e. for 80m 1,3 or 5 x
30.9' x Vf which for their 300 line is 0.88. DXE recommends a 1:1 balun
at the end of the 300 ohm, suggesting that tuners handle high Z mismatch
better than low Z. I did check the Z at that point with an AIM 4170 and
concluded that a 2:1 balun was better in my case (dipole at 70'). A
100ohm impedance seemed to be a better average value across the bands of
interest on the 4170 SWR graph.
Your 160m inverted V will likely be omnidirectional vs the classic
narrowed dipole pattern and gain of "two half waves in phase" when on 80m.
b.t.w. this dipole kit is very well made and a bargain as it includes
the very clever patented end and center insulators, 135' of #14 antenna
wire, SS hardware, and 100' of 300ohm ladder line all for $50. Compare
that to the other junk dipoles around. The 160m is ten bucks more.
DXE-WA-135 and DXE-WA-260. (I have no connection to DXE).
Grant, KZ1W
On 11/6/2010 1:45 PM, Steve Hunt wrote:
> An EZNEC model of an Inverted-L with 60ft vertical and 77ft horizontal
> gave me a resonant feedpoint resistance of 32 Ohms at 1.8MHz when the
> ground loss resistance was 13 Ohms. That same antenna at 3.5MHz had a
> feedpoint impedance of 6000-j1000 Ohms.
>
> That's to be expected - the wire is close to a half-wave long on 80m.
>
> 73,
> Steve G3TXQ
>
> On 06/11/2010 20:21, Bill Gillenwater wrote:
>> If you have a feedpoint impedance of 32 ohms on an inverted "L" for 1.8 mhz,
>> can anyone venture a guess as to what the impedance would be of the same
>> antenna at 3.5 mhz ??
>>
>> thanks, Bill
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