[Elecraft] center fed dipole

[Lyn Norstad]

Hi Wes -

Yes, you could model this ... and I did do that first to get a gut check on the concept.  But as I said, I don't rely 100% on the modeling.  I want to see real world measurements.  I used an antenna analyzer to read the impedance (comprised of resistance, inductive reactance and capacitive reactive).  Those are the figures quoted.

The gain on 80 meters is right where I want it, in a N-S pattern.  My purpose in building it for 3.5 MHz N-S was to put the best possible NVIS signal into our state capital (Springfield, IL) on 80 meters for EmComm purposes.  It does that job very nicely, and I find myself able to do so when others around me are having difficulty.

It has decidedly different patterns on other bands, but none are particularly objectionable.  If you'd like to see the plots, as imaged by EZ-NEC, they're on my QRZ page (scroll down).

In the 2 years it's been up, I have logged 131 contacts in your area - Tucson - on 160, 80, 60, 40, 30, 20, 17, 15 and 12 meters.  

It's the only antenna I have, and it works well.

73
Lyn, W0LEN


-----Original Message-----
From: elecraft-bounces at mailman.qth.net [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Wes
Sent: Monday, November 01, 2021 5:39 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] center fed dipole

I have some observations and/or questions

First I think you could model this.

Second, how did you measure these impedances?  You quote them as resistive, are 
they?

Is that 5 dB gain in a useful direction? What about the other bands?

Wes  N7WS

On 11/1/2021 7:39 AM, Lyn Norstad wrote:
> 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
>
>

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