[TowerTalk] balanced line loss on a mismatched antenna

Tue Feb 28 09:40:17 PST 2012


No way 44 foot long elements give - j17.7 reactance at 7.0 MHz.

Dave   AB7E


On 2/28/2012 9:18 AM, Paul Christensen wrote:
> Gary,
>
> I imported the same model into 4Nec2 and scaled the frequency to 7.0 MHz.  Feedpoint Z is 4.6-j17.7.  Your Z values look quite extreme (also see notes below about feed Z for the 8JK).
>
> Using Zo=450-ohm line, two points result in minimum SWR(50) at the line input.  The first occurs at 65 ft where SWR(50) is 7:1 and a total loss of 1.8 dB.  The second point will occur a half-wave more at 130 ft. where SWR(50) is 5:1 and total loss is 3.0dB.  VSWR is lower, but it's the result of additional loss due to SWR.  So, loss is higher, but SWR is lower due to added loss.
>
> Me, I would add an L network at the feedpoint.  First, look at transforming the low feedpoint Z to 450+j0.  What L network values?  A high-pass can be made with 800 pF C in series at the feed, followed by a shunt of 1 uH of L.  L would be a small turn or hair-pin of wire.  That combination results in a perfect match from 4.6-j17.7 into 450+j0.  Total L network loss is only 0.23 dB with reasonable component Q. Finally, let's see what 150 feet of loss is with the L in place.  Well, SWR(450) is 1:1, and line loss is now only 0.1 dB but SWR(50) is 9:1 at the line input.  So, a tuner must be used between the Tx and line input.
>
> How about a deliberate mismatch at the feedpoint to some intermediate Z value to allow for line length trimming?  We can still use an L network, but now convert 4.6-j17.7 into 50+j0. That's 0.37 uH across the feed, followed by 700 pF of series C.   At 127 feet of line, SWR(50) is now 1.1:1 and line loss is respectable at 0.4 dB (+0.23 dB of network loss = 0.63 dB total) and -- no tuner required!
>
> FYI - Here's the comment section for the 4Nec2 file concerning the 8JK:
>
> "Originally designed by John Kraus, W8JK in about 1940, this antenna has some interesting properties. It by two closely spaced elements driven out of phase.
> Although the fields from the elements don in any direction, gain is nonetheless achieved because of lowering of the radiation resistance due to mutual
> coupling. And lower it is -- note the feedpoint impedance of only 4.74 - j19 ohms. Compare this to a single
> element. The lower resistance results in heavier current, hence greater field strength, for a given power input. The
> difficulty is that system losses can quickly eat up the gain. Making this antenna from #12 copper wire (try it --
> and include wire loss) drops the gain about 0.65 dB, not too bad. But great attention must be paid to losses in
> matching networks. And losses rapidly increase in significance as the spacing is made closer than the 0.1
> wavelength of the example. When mounted low (0.25 wavelength for the example), W8JK-type antennas have a
> lower radiation angle than many other horizontal antennas due to the inherent lack of high-angle radiation. As an
> interesting exercise, save the pattern for later comparison. Then delete the second source, making the
> antenna into a Yagi. Note the increased gain. Even though the "takeoff angle" is higher, the Yagi gain is as good or
> better even at lower angles. In addition, the feedpoint impedance has increased to a much more manageable value. On
> the other hand, the W8JK will retain its performance over a much greater frequency range than the Yagi."
>
> Paul, W9AC
>
>    ----- Original Message -----
>    From: Gary Slagel
>    To: Paul Christensen ; TowerTalk
>    Sent: Tuesday, February 28, 2012 10:11 AM
>    Subject: Re: [TowerTalk] balanced line loss on a mismatched antenna
>
>
>    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 at arrl.net>
>    To: Gary Slagel<gdslagel at yahoo.com>; TowerTalk<towertalk at 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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