[Elecraft] center fed dipole

Mon Nov 1 10:39:48 EDT 2021

In my book, there are too many variables in any given installation to rely 100% on any "standard" convention, including modeling.

When I built my Extended Double Zepp (a 360 foot long, 1.25 wavelength center fed dipole) with a design frequency of 3.5 MHz (and approx. 5 db gain), I measured the impedance at the "shack end" of the feedline (160 feet of 600 ohm OWL from https://www.trueladderline.com/ ) at multiple points on each band.  Actual readings ranged from 323 ohms at 1840 KHz, 536 ohms at 3573 KHz, 598 ohms at 5357 KHz, 249 ohms at 7074 KHz, 371 ohms at 10136 KHz, 344 ohms at 14074 KHz, 260 ohms at 18100 KHz, 50 ohms at 21074 KHz, 182 ohms at 24915 KHz and 142 ohms at 28074 KHz (note tuner runs in 'bypass' mode on 15 meters where it is roughly 7.5 wavelengths).

I'm, feeding that thru a hybrid balun which is 4:1 on the antenna side, and 1:1 on the shack end (Balun Designs #4116-T).  Approximately 15 ft of RG-8x runs from the balun to the KAT500.

The KAT500 gives me a 1.00 match on all those frequencies (standard FT8 frequencies) and generally less than 1.5:1 at any point on all bands 160 thru 10 meters whether I'm running 10 watts or 500 watts. 

The weak spot in the chain is the KPA500 which will not allow the KAT500 to tune properly if the amp is in "Operate" mode (it did when it was new, but that's another issue).

Every installation is different, but this combination works as well for me as I could hope and results bear that out.

73
Lyn, W0LEN

-----Original Message-----
From: elecraft-bounces at mailman.qth.net [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Ray
Sent: Sunday, October 31, 2021 4:04 PM
To: Bob McGraw; elecraft at mailman.qth.net
Subject: Re: [Elecraft] center fed dipole

Your Guessing !
Did you Model it ???
Then you will know……..
Ray WA6VAB K3

Windows 0

From: Bob McGraw
Sent: Saturday, October 30, 2021 6:17 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] center fed dipole

I don't understand the use of the 4:1 balun.  On 80M the feed Z of that 
antenna is more likely 25 to 50 ohms.  The 4:1 balun would present a 
impedance to the RG-213 of 6.5 to 12.5 ohms.  You would be much better 
with a 1:1 balun configuration.  Remember one is not matching the feed 
line impedance but is matching the antenna impedance.

The ideal use of a 4:1 balun is with a folded dipole antenna which 
presents a 200 to 300 ohm impedance.  Now the feed line Z is much closer 
matched and the 4:1 balun gets the Z to the coax in the 50 to 75 ohm 
range.  Much more efficient.  This of course is a single band antenna.

See https://www.dj0ip.de/balun-stuff/  for good, accurate, and reliable 
information on baluns and their usage.  Most hams have it wrong as well 
as most manufactures of antenna tuners who include a 4:1 balun.

73

Bob, K4TAX

On 10/30/2021 5:07 PM, elecraft-request at mailman.qth.net wrote:
> Message: 11
> Date: Fri, 29 Oct 2021 18:25:44 -0700 (PDT)
> From: MIKE ZANE<n6zw at comcast.net>
> To:"Elecraft at mailman.qth.net"  <Elecraft at mailman.qth.net>
> Subject: [Elecraft] center fed dipole
> Message-ID:<1705203692.528740.1635557144218 at connect.xfinity.com>
> Content-Type: text/plain; charset=UTF-8
>
> I have no problem with a 130 ft dipole fed with 25 feet of 450 ohm ladder (window) line to a 4:1 4K balun with 15 feet of RG-213 to my MFJ 989C tuner. I have run from 5w to 1 KW with it....no problems at all.  n6zw

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