[TowerTalk] Re: Coax balun question

[Bill Coleman AA4LR]

On 5/5/00 3:27 PM, Chuck Hartley at hartley@smart.net wrote:

>I was wondering - what is the minimum bend radius for RG-213? Winding
>it on 4 inch form must be about the minimum you can do.

Hmm. A quick search on the net yeilds a K1TTT page that suggests the 
minimum bending radius for RG-213 is 100mm 
<http://apocalypse.berkshire.net/~robbins/technote/coaxloss.html>. That's 
just under 4 inches to us metric-deficient Americans.

I wouldn't worry terribly about bending a solid-dielectric coax. It's the 
foam or air dielectric stuff which has trouble with center-conductor 
migration that can short out.

> How about
>using some of the smaller teflon dielectric coax like RG-303 instead?
>Other than costing 3-4 times as much as 213, would it be better from 
>a power handling and weight standpoint?

Well, costs just about nail it on the head. You also have to find the 
stuff. The main advantage of the coiled-coax balun is that it is 
extremely inexpensive, while maintaining effectiveness.

> Would the smaller diameter
>(of the coax itself) require more or less turns for the same reactance?

If wound on the same form, with the same spacing (the smaller coax would 
wind tighter, so you'd have to space the turns some), it would be the 
same. But even if you changed the spacing slightly, the exact value of 
the choke isn't critical. If in doubt, run the formulas and calculate the 
inductance of the RG-213 version, and add or remove turns to match for 
smaller coax.



Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@radio.org
Quote: "Boot, you transistorized tormentor! Boot!"
            -- Archibald Asparagus, VeggieTales


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