[Antennas] Quad Loop Termination

[npalex at aol.com]

Group,
I would like to add my comments on this frequently asked question regarding 
 the termination of the non-active quad driven elements in a multi-band  
array.    There is no simple answer as there is no simple defined  loop 
termination.  
 
If it were possible to disconnect the feedline at the driven element then  
it might be possible to create an "Open" loop or a "Closed" loop.  But in  
most installations the feed line is permanently attached to the feed  point 
through an odd quarterwave on some or all bands, or in some  cases a 
half-wave length at the chosen band frequency.  
 
Remember these feed line lengths look like a specific wavelength only at  
the frequency of that band.  The issue is what do they look like when  
excited by a harmonically related band.  In a tri-band quad you can be  on twenty 
meters, but both the 15m and 10m loops are being excited at that  frequency 
also.  If we want those other band loops to have no circulating  current 
then we want the loop to be terminated in a very high impedance.   Therefore it 
is necessary to calculate the Z at the loop based on that feed line  
termination (open or shorted), its electrical length at the excited  frequency.  
It will be some complex value but not likely a very high  impedance.   You 
have to compute all the variations for each  band/frequency of operation and 
non active loops, both shorted or open at the  antenna selector switch to 
determine the load presented.  The solution will  be effected whether the coax 
shields are all commonly connected or switched open  (DPDT switch, isolated 
chassis connections).
 
I have never resolved all the variations, but the solutions I did showed  
that some unique length of coax feed line for each band would reflect a 
nearly  open or high impedance termination.  This gets to be a lot of work on a 5 
 band quad.
 
Conclusion was to feed each loop for best match on transmitting, and not  
worry about the interaction (usually some form of pattern distortion, or 
reduced  F/B or side nulls.   In general I recommended setting up the remote  
switch to be open circuit.
 
Norm W4QN
(formerly of Cubex Quads)
 
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Message:  1
Date: Thu, 17 Dec 2009 12:54:14 -0800 (PST)
From: Tom Horton  <k5iid at sbcglobal.net>
Subject: Re: [Antennas] Quad Question
To: WX5L  <wx5l at charter.net>, Antennas at mailman.qth.net,  w1tjl at arrl.net
Message-ID:  <431832.45775.qm at web82107.mail.mud.yahoo.com>
Content-Type: text/plain;  charset=iso-8859-1

That is one of the ongoing debates... I never have  grounded them or even 
opened the braid and I have always had fantastic results.  
NOW that does not mean it wouldn't work better with some other arrangement  
but how much better is the question.
?Tom  K5IID 
Hillsboro, TX  

- On Wed, 12/16/09, Tom <wb1cby at gmail.com> wrote:


From:  Tom <wb1cby at gmail.com>
Subject: Re: [Antennas] Quad Question
To:  "WX5L" <wx5l at charter.net>, Antennas at mailman.qth.net
Date: Wednesday,  December 16, 2009, 11:22 AM


According to LB Cebik, they should be  opened.? Practically he said to 
feed them with a 1/4 wavelength (electrical)  of coax and open th end 
(bot braid and center conductor disconnected) -  leaving the braid 
grounded grounds the loop.? I built a 6 band version on a  Gem Quad 
Spyder frame and it works great! Used DPDT relays for each band and  
wired them to open both sides of the coax when de-energized.

73,  Tom


On 12/16/2009 12:09 PM, WX5L wrote:
>  Hello,
>
> Is it best to have unused loops of a 5 band quad grounded  OR
> isolated(floating) through an antenna switch.

>  Thanks,

> Randy
>
> WX5L