10 years ago I tried exactly what Matt outlined below. Two 40 meter elements
using a Comtek 4 square box. The Comtek manual had a sheet on how to do it -
including the 50 ohm phasing lines. The VSWR was OK (so is a dummy load!), but
the performance was not very good. F/B was less than 10 dB. The gain seemed low
as well. I went to EZNEC and modeled the entire system and the results
confirmed my field experience. The configuration of a Hybrid with the dump
power load does not work properly in a 2 element system when mutual coupling
between the elements are taken into account (which is where the original Comtek
documentation failed). Notice that current Comtek 4 square box documentation
does not include a two element installation option because it does not work.
This is why DXE now has the DVA product. Designed specifically for two vertical
elements - including the effects of mutual coupling. No dump power load. Simple
and easy to install - very effective. If you worked the Tromelin or Juan de
Nova DXpeditions in the last 18 months - they used a DXE DVA on 80 meters.
Their signal was outstanding.
By the way - 4 square boxes and the DXE DVA all require full size quarter
wavelength elements to work properly. Short loaded elements have different
mutual couplings which will result in compromised performance
The DXE-TFS boxes cannot be used with two elements for the same reason as the
Comtek.
73
Tim K3LR
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Jorge
Diez - CX6VM
Sent: Wednesday, November 30, 2016 7:17 AM
To: Matt
Cc: Tower Talk
Subject: Re: [TowerTalk] FW: 2 Element phased vertical array using Comtek
Foursquare Box
Hello
you dan do the same with DXE-TFS4-80 box?
thanks
Jorge
CX6VM/CW5W
2016-11-30 0:57 GMT-03:00 Matt <maflukey@gmail.com>:
> The Comtek model ACB-4-A series units could be used to drive a 2 element
> phased array by connecting the 2 verticals separated by a quarter
> wavelength on ports #1 and #4 of the hybrid quadrature unit. Leave ports
> #2 & #3 disconnected but do connect a 50 ohm dummy load to the dump port.
> You will need to feed the antennas with 1/4 wave electrical length 50 ohm
> coax instead of the 75 ohm used in the 4-square configuration - no
> impedance transformation is required because only one antenna will be
> driven from the opposing sides of the hybrid bridge. You may also want to
> call Dx Engineering or Comtek and just confirm this info is still valid
> with the current series units but as far as I know it's essentially the
> same circuit.
>
> Hope this info is helpful. Good luck on you project.
>
> 73
> Matt
> KM5VI
>
>
>
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
> Marty s
> Sent: Monday, November 28, 2016 1:27 PM
> To: towertalk@contesting.com
> Subject: [TowerTalk] 2 Element phased vertical array using Comtek
> Foursquare Box
>
> Greetings Everyone.... hope CQ WW CW treated you well,
>
> I have everything I need to erect a 40m foursquare on my tiny urban lot in
> MA. It looks like I won't get it properly finished in time for this
> winter, but would like to at least get some gain out of the two verticals I
> have up right now, to get 2 directions of endfire. Can a normal Comtek box
> with no modification be used to handle this, or is it more effective for me
> to just phase the two verticals with a homebrew box relying on 1/4 wave
> 75ohm phasing lines?
>
> Thoughts, opinions, concerns, questions?
> _______________________________________________
>
>
>
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>
>
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>
--
73,
Jorge
CX6VM/CW5W
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