We're not discussing a choke operating at very high frequencies,
we discussing its performance at 1.8 or 3.5 MHz. At these frequencies,
a few feet of 50 ohm transmission line such as RG-142 or RG-400
connected to the end of a 75 ohm line has a tiny -- nearly immeasurable --
effect.
Using this on-line transmission line calculator:
www.iz2uuf.net/wp/index.php/transmission-lines-calculator
Two feet of RG-142 connected to a 75 ohm load has an input impedance
of 75 -2j ohms at 1.8 MHz and 74.8 - 4J at 3.5 MHz. It would be very
difficult to measure such a tiny impedance transformation, much less
trying to measure its insignificant affect on the performance of a
4-square array.
Even a four foot length of RG-142 transforms the impedance to only
74.8 - 4j ohms at 1.8 MHz and 73.9 -7.5j at 3.5 MHz
73
Frank
W3LPL
----- Original Message -----
From: "Gedas" <w8bya@mchsi.com>
To: rfi@contesting.com
Sent: Wednesday, January 22, 2020 2:59:21 PM
Subject: Re: [RFI] Choke questions
Another Teflon based coax similar to 50-ohm RG-142 and 75-ohm RG-179 is
75-ohm RG-187.
Gedas, W8BYA EN70
Gallery at http://w8bya.com
Light travels faster than sound....
This is why some people appear bright until you hear them speak.
On 1/22/2020 1:35 AM, Jim Brown wrote:
> On 1/21/2020 8:02 PM, John K9UWA wrote:
>> Is there a 75 ohm equivalent to RG400 coax?
>
> Not that I know of, although are there's at least one sort of like
> RG142. KZ1W sent me several miniature coaxes to evaluate for chokes
> for TX 4-squares, the most useful of which seems to be RG179. It's
> addressed in the Cookbook.
>
> 73, Jim K9YC
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