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## Re: [TowerTalk] balanced line loss on a mismatched antenna

 To: Paul Christensen Re: [TowerTalk] balanced line loss on a mismatched antenna David Gilbert Tue, 28 Feb 2012 10:40:17 -0700 mailto:towertalk@contesting.com>
 ``` 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 > To: Gary Slagel; TowerTalk > 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 > > > _______________________________________________ > > > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk > _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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