Tom Rauch wrote:
>
>Hi Ian,
>
>> The term "UHF" is a complete misnomer in discussing these connectors. They
>> are strictly HF devices. VSR above around 140 MHz is too high. The better
>> types use PTFE, bakelite or similar insulation (won't melt when heated
>> with a soldering iron).
>
>The impedance bump in a UHF connector is confined entirely to
>the area of the female about 1/2 inch long, assuming a hood is
>used on the female's rear. The impedance of that bump is about
>30-35 ohms.
>
>The VSWR error from using a single connector, assuming the 30
>ohm impedance and a VP of .5 is 1.028:1 at 50 MHz.
>
>It is only 1.25 at 400 MHz!
>
>That's why you find UHF connectors used on landmobile radios up
>to 400 MHz and higher. The impedance bump, and loss, of a UHF
>connector can be pretty much ignored up to 250 MHz, and often is
>negligible above 250 MHz if the connector has partial air dielectric
>in the female section.
That's certainly true, and of course the UHF connectors are much more
"monkey proof" than the N.
The SWR problem is not with individual connectors but with multiples. On
the back of the radio, the impedance bump can be compensated by design,
for both the transmitter and the receiver. The problems arise further
down the line if there are several of these connectors separated by odd
lengths of coax. Murphy says the reflections will add in phase, and then
you're into using special lengths of coax in order to get the SWR back
under control.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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