Dave,
The 8JK EZNEC input file is for 0.466 wavelength elements and 1/4-wave
spacing. That is going to produce the results shown. Going back to Gary's
first post, he does indicate 44 ft elements at 7 MHz, but according to
Cebik, the 44 ft elements should be used only on a 10m-30m 8JK array.
I'll confirm your 2.5-j600 result with 44ft elements. For the amount
shortened, it does look right.
Paul, W9AC
----- Original Message -----
From: "David Gilbert" <xdavid@cis-broadband.com>
To: "Paul Christensen" <w9ac@arrl.net>
Cc: <towertalk@contesting.com>; "Gary Slagel" <gdslagel@yahoo.com>
Sent: Tuesday, February 28, 2012 12:40 PM
Subject: Re: [TowerTalk] balanced line loss on a mismatched antenna
>
>
> 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@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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