[TowerTalk] PL-259's at VHF

[Ian White, G3SEK]

MAERTZ, ALLISON R wrote:
>
>Greetings:
>
>I hope this is the right reflector for this question.  
>
>I am installing a VHF/UHF system with a Kenwood TR-7850 and two M^2
>yagis.  Feedlines are 65 feet of 9913 with 9913 F jumpers at the rotator
>to allow rotation/elevation.  Of course I'll use 'N' connectors for the
>UHF connections except at the rig where Kenwood has an SO-239 (this
>creates five N connections plus the PL-259/SO-239 at the rig for a total
>of six connections).  On the VHF side, I have the same number of
>connections, but...
>       
>       My questions:   
>
>       a.  Is the total signal loss at 144 mhz through the standard
>PL-259's significant with those six connections,
>suggesting that I should use N connectors on my feed at 144 mhz as well
>as at 436 mhz? 
>       
>       (I've come across two schools of thought here relative to using
>standard PL-259's at 144 mhz, as well as varying numbers on    the db
>loss at these connectors at this frequency.) 
>
The problem is more than the dB loss of individual connectors. The loss
of a PL-259 is mainly due to a small impedance bump - it isn't a
constant-impedance 50 ohm connector. The reflection from one PL-259
would be too small to cause a problem at 144, but you will have six of
them, plus the reflection from the antenna itself.

These small reflections will all combine together to determine the
impedance that your transceiver sees at the bottom of the cable.
Depending on the electrical lengths of cable between them, the
individual reflections can either add, cancel or (more likely) something
in between. You may be lucky, and finish close to 50 ohms at the
transceiver, but it's more likely that these small reflections could add
up to quite a high VSWR.

>       b.  Should the connection at the rig be modified at VHF or UHF
>with a female N connector or an N->PL-259 connector in         order to minimze
>loss or is it insignificant there?  (I believe Yaesu has 'N' connectors
>on their rig)

I'd only change the connector on the transceiver as a very last resort.
After all, its only one out of a total of six.


73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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