I loaded your antenna description (two 44 foot long wires spaced 16 feet
apart at an assumed a height of 70 feet) into EZNEC 5+ and got source
impedances of 2.5 - j600 ohms.
The application TLW is available free with the ARRL Antenna Book. If
you load your parameters (150 feet of 450 ohm ladder line with a load
impedance of 2.6 - j400) you get an SWR of 248:1 and a theoretical loss
of 7db. If you load the values I got (2.5 - j600) you get an SWR of
375:1 and a line loss of 8.7 db. Either way it's an ugly proposition
and you could expect things to be a lot worse when the ladder line gets
wet or dirty.
If you use low impedance coax the high capacitive reactance is going to
also give you a high SWR although you could more easily tune that out at
the antenna with a simple inductor, and you could tune everything to 50
ohms with a network. TLW will also calculate that for you (just set
the line length to zero) and it says that an L-network tuner will
itself have a loss of about 4.5 db (assuming Q=200 for the inductor and
Q=1000 for the capacitor).
I really don't see the point of this configuration at all. You're
talking about putting up a W8JK antenna to get maybe 3 or 4 db gain over
a dipole only to throw that same amount away in a tuning network at the
antenna if you use one, or twice that amount in feedline losses if you
don't. You'd be way better off with just a dipole (EZNEC says 22 - j600
ohms for a 44 foot element) and either:
a. coax feedline with an L-network at the antenna. TLW says 1 db
network loss plus 0.5 db line loss for RG-8.
b. ladder line with a tuner at the shack. TLW says 2.4 db line loss
plus whatever loss you have in the shack tuner. The loss in the tuner
in the shack might be fairly small since TLW says that the impedance at
the shack would be a fairly manageable 6 + j45).
Dave AB7E
On 2/28/2012 8:11 AM, Gary Slagel wrote:
> Thanks Paul,
>
> I've attached the Source Data output below. This, I believe, is showing me
> SWR and impedance at the feedpoint of each element. I'm not sure if EZNEC
> gives me a way to see the SWR at the single feedpoint that I'll use to feed
> these two feedpoints but I assume it will be equally as bad.
>
> I'm using the standard ARRL W8JK model that comes with the demo version of
> EZNEC in the ARRL antenna book. I modify that model to look like what I want
> to build and as long as I don't save it the software performs like the full
> version of EZNEC. I'm over my head with antenna modeling as soon as I move
> past modeling a dipole but I forge ahead and try and get as much useful
> information as I can from it.
>
> The question still is, if I get the antenna working properly how much loss
> will I see on the feedline. Thanks much for your help.
>
>
>
> EZNEC ARRL ver. 4.0
> 2/28/2012 7:53:59 AM
> --------------- SOURCE DATA ---------------
> Frequency = 7.05 MHz
> Source 1 Voltage = 397.6 V. at -89.63 deg.
> Current = 1 A. at 0.0 deg.
> Impedance = 2.597 - J 397.6 ohms
> Power = 2.597 watts
> SWR (50 ohm system)> 100 (75 ohm system)> 100
> Source 2 Voltage = 397.6 V. at 90.37 deg.
> Current = 1 A. at 180.0 deg.
> Impedance = 2.597 - J 397.6 ohms
> Power = 2.597 watts
> SWR (50 ohm system)> 100 (75 ohm system)> 100
> Total applied power = 5.193 watts
>
>
>
> Gary Slagel
> KT0A
>
>
>
>
> ________________________________
> From: Paul Christensen<w9ac@arrl.net>
> To: Gary Slagel<gdslagel@yahoo.com>; TowerTalk<towertalk@contesting.com>
> Sent: Monday, February 27, 2012 4:48 PM
> Subject: Re: [TowerTalk] balanced line loss on a mismatched antenna
>
>> "I think I can tune the antenna with the feedline and get it down to a more
>> reasonable level, maybe 10 to 1, at the tuner end. Then the tuner will tune
>> it to resonance so the xmttr can put its full load into.
>> Question is, since the swr is 10 to 1 at the radio end of the feedline but
>> 100 to 1 at the antenna end of the feedline, am I going to see the 3.5 db
>> loss from a 100 to 1 swr or the .5 dbi that a 10 to 1 swr would give me."
> Gary, you can vary the impedance at the input end of the line, but not the
> SWR. The SWR on the 450-ohm section will remain nearly constant over your
> 150 ft run, even with length trimming.
>
> In EZNEC, what is the Z of the antenna you're modeling at the operating
> frequency in an R+j format?
>
> Paul, W9AC
> _______________________________________________
>
>
>
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