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Re: [TowerTalk] 80m 4Square Comtek box

To: "'towertalk'" <>
Subject: Re: [TowerTalk] 80m 4Square Comtek box
From: Jim Lux <>
Date: Thu, 18 Nov 2004 12:03:50 -0800
List-post: <>
At 02:23 PM 11/18/2004 -0500, Tom Rauch wrote:
> Actually, there is enough information in ON4UN's book for
> competent individual to build a hybrid style (quadrature)
> phasing unit without much more than an MFJ-259B.  Of
> a good scope and current probes would help if one wanted
> optimize the unit.

By the time most people did all that, they would be better
off just buying the Comtek. Retail parts aren't cheap or
easy any longer.

I'll second that...
Let's see..

Knowing the values is easy.  for 50 ohms, 3.8MHz, you want L=2.09 uH, C=419pF
The schematic is simple, too.

Then, you can engage in a few hours of design time to find an appropriate core and design the inductor. (remember, it has to handle a kilowatt)
The caps are catalog parts (although you need to make sure they have the right current ratings, etc.), so it won't take long to find them.
Then, you'll need to select some appropriate relays, a decidely non-trivial process, since RF relays that handle a kilowatt are quite pricey, so you're going to want to use some other kind.

Order everything (after rummaging through the junk box) you need.

I'd wag the parts cost at around $100-200 (but it could easily be more... suitable non-surplus relays are probably $10 each in the catalog, the caps will be fairly inexpensive, cores are a couple bucks, but you have to consider things like enclosures, connectors, etc.)

Wait 2 weeks (because you can't really work on it during the week).

Now, spend some time winding that inductor, figuring out how to assemble it, put it into a box, etc. It's probably a nice 2 day job. And, if you have the time and tools, it would be enjoyable.. a nice combination of mechanical engineering and electronics.

But all the same, if you paid yourself something like $8/hr, you'd probably spend 20 hours on the whole project, so the labor cost is $160.
That's a pretty unrealistic labor cost. Either you have the time to spend on it, in which case the time is free, or you're busy with lots of other things, in which case the time is infinitely expensive.

Hmm.. or, just buy the Comtek box for $350. Not a bad deal.


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