75 ohm connector in VHF circuit

[Durcott]

Hi Folks,

I've never been happy with my Shore-Line Compact VHF set. The range is not good at all.

There could be a number of reasons I guess - but when I was preparing for unstepping the mast, I found that the bulkhead co-ax connector was - or at least looks very much like - a 75 ohm socket. (Very thin centre contact on an 'N-Type' connector)

Theoretically bad - does anyone have a view as to how bad in practise?

Thanks

Jeff Adams

 

[tome]

If it's an N type it will be 50 ohm impedance so shouldn't be a problem - typical insertion loss about 0.25dB per connector

 

[Birdseye]

If your radio has poor range but still does work, then its a pound to a pinch of whatsit that the problem is the aerial and its cable rather than the set. Modern radios tend to either work or fail altogether. The 50 ohm cable to the aerial needs to present as little resistance as possible so you need good connections and nice thick un corroded wire. The effect of a 75 ohm connector would be to reflect part of the signal back to the radio itself ie increase the Standing Wave Ratio,

If you can get your hand on one, get and SWR meter and put it between the radio and the aerial. Ideally you need an SWR of between 1 and (say) 1.3. Up in the 2's and above you have definite aerial problems.

And make sure the 12v cable to the radio is good - high resistance here can cause the voltage to drop when the demand increases on transmit and then the power will go down.

 

[William_H]

No in practice you would never notice the losses caused by a 75ohm connector in a 50 ohm cable. Providing it is a good connection. The losses increase with frequency and at VHF would be quite small. However your problem is probably in the antenna or cable. So get a VSWR meter if you can. Or substitute another antenna and cable for test.

VSWR mters here in Oz used to be all the rage for CB radio nutters.
Now 27meg CB seems to have died. The electronics houses seem to have stopped stocking the cheap meters and it seems the only source for cheap ones is second hand at junk markets. Thats where I got mine.
However it is not a huge project to build one. Google probably has designs. The design I remember seeing had a small section of coax cable with connectors each end to connect into the aerial circuit. Two this pieces of enamelled wire were threaded under the braid paralell to the min coax conductor. Thes wires were terminated with 50 ohm resitors one at left end the other at the right end. The other ends of the wires went to a small diode and bypass capacitor and the ends then went to a switch thence to a meter. The switch connecting the meter to one wire or the other one psoition measuring forward power the other position measuring reverse power. I don't think I have a copy of the design and memeory is not so good but PM me if you are interested..... olewill

 

[stevebirch2002]

I agree but depends on quality of "N" type and frequency. 2 metre range (Marine Band) should noit have an affect. Best to get an SWR meter to check cable and aerial. Not expensive.