[TowerTalk] Comtek 4sq box question

Jim Lux jimlux at earthlink.net
Sun Aug 20 13:50:04 EDT 2006

At 10:03 AM 8/20/2006, ON4IQ wrote:
>I got a 2nd hand 4sq box (Comtek) a while ago and decided to put it on the 
>The 4 antenne ports are terminated with 50Ohm suhner dummyloads, the load 
>port goes to the network analyser and the xmtr port to the analyser aswell.
>I inject a 3.5 to 4MHz signal into the comtek box and see on the load port 
>(where your dummy load needs to go) a level -7.5dB down on the xmtr level.

I assume this is an 80m box?  Isn't it the "collins" hybrid (2 inductors, 2 
capacitors) plus a transformer to split the center elements feeds?

One might want to try checking Tx->port 1,2,3,4 with loads on the other 
ports, just to get a feel for it.

One might try disconnecting one load at a time, and seeing if the reflected 
power into the load changes by what might be expected?

There's some work out there on measuring N-port devices with a two port 
analyzer.  While the idealized case is to terminate the N-2 ports in 50 
ohms, there are some accuracy issues, particularly if your 50 ohm loads 
really aren't 50 ohms.  I forgot the authors' names (Tippet? Speciale?), 
but there's a technique where you terminate them into a deliberate mismatch 
(something like 2:1) and algebraically reduce it that gives better accuracy.

>This seems pretty high to me as I found in on4un's book notion of -21dB as 
>a value for the dumped power when terminated in 50ohm.
>Is there reason to suspect the seller sold me a bad unit?
>73  Johan
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