(Re)fitting a DSC radio - what resistance across VHF aerial?

[Danny Jo]

Flushed with the success of replacing Freestyle's Eberspacher without breeching any bulkheads (by the rather complicated expedient of cutting the terminals off the old and the new wiring looms, and fitting the new terminals to the old wiring by soldering and sealing with heat-shrink tubing), I decided to have a go at moving my VHF.

I've never liked it in this position, because it creates an annoying obstruction on the chart table, and because you have to twist over the table to get a clear view of the keys. Rather than attempt the risky manoeuvre of enlarging the holes through which the wires emerged to accommodate the plugs, I went for the even riskier manoeuvre of cutting the wires.

The remote mike/control wire gave me a metaphorical shock, one of the "cores" being rather thicker than the others owing to it being a tiny coaxial cable. It took ages to . . . oh, never mind, I'll get straight to the question:

Having failed in my attempt to join the two cut ends of the VHF aerial (I could receive, but not transmit) I fitted a new coaxial plug to the VHF aerial, and it works a treat. Checking the resistance across the aerial (when not attached to the radio), I was surprised to find that the resistance was only 2.3 ohms (or at least it read 02.3 on the lowest setting, 200). Does this mean I have a short between the shield and the core? And if the answer to this question is yes, why does it send and receive loud and clear?

Also, I've not yet worked out why the set cannot receive the GPS position, in spite of several attempts at resoldering the audio-type jack plug.

Moral: amateur electricians should not try this manoeuvre at home.

 

[Heigaro]

That's an interesting navigation instrument on the chart table. It looks as if it doubles up as a bottle-opener. Haven't seen those in the Force 4 catalogue - more of a Nauticalia item methinks!

/forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif

 

[VicS]

[ QUOTE ]
I was surprised to find that the resistance was only 2.3 ohms (or at least it read 02.3 on the lowest setting, 200). Does this mean I have a short between the shield and the core? And if the answer to this question is yes, why does it send and receive loud and clear?


[/ QUOTE ] You would expect a low resistance. Not because the inner and outer conductors are shorted but because they have a coil connected accross them at the aerial end.

 

[theforeman]

are you planning a trip north this summer ?

 

[Engage]

A multimeter will usually attempt to pass a direct current by presenting a DC voltage to the probe tips when set to ohms mode. This will yield a DC resistance. However, your radio will output AC, alternating current. The characteristics of a circuit's resistance under AC is known as impedance and will usually be different to the equivalent DC resistance. Thus it is normal to have a low DC resistance for a wire coil load that has a higher AC impedance. This does not mean you do not have a short circuit though. I would expect a passive antenna to have an open circuit characteristic as is mine. Clear as mud?

 

[john_morris_uk]

[ QUOTE ]
A multimeter will usually attempt to pass a direct current by presenting a DC voltage to the probe tips when set to ohms mode. This will yield a DC resistance. However, your radio will output AC, alternating current. The characteristics of a circuit's resistance under AC is known as impedance and will usually be different to the equivalent DC resistance. Thus it is normal to have a low DC resistance for a wire coil load that has a higher AC impedance. This does not mean you do not have a short circuit though. I would expect a passive antenna to have an open circuit characteristic as is mine. Clear as mud?

[/ QUOTE ]Yes - and a good attempt at explaining the problem. However the answer to the question is as VicS intimated. Most Marine VHF's have a loading coil built into their base and to all effects and purposed present a short at DC. Therefore a reading of 2.4 Ohms might be about right. (You could always ask to measure a new antenna's DC resistance in the chandlers or phone the manufacturers and ask them for the correct figure?)

By the way can you name a VHF marine antenna that ISN'T passive? Your antenna showing open circuit is unusual. Most are 5/8th whips with bottom loading coils.

A VSWR meter wwhich shows a reading of less than about 2:1 will give you some more confidence that all is well, but remember that a lossy cable (ie one full of water) will also give a low VSWR figure. VSR meter's are simple devices that you put in line with the coax and give you and idea of the impedence match (or mismatch!).

 

[simonfraser]

re the gps bit, IF you have joined the cable colors up correctly, you may have disturbed a connection further up th chain, possibly pulled it out / off the gps side ??
my gps has a function to either send or not send nmea data out, has this been changed ???

 

[Danny Jo]

Thanks everyone for the useful and reassuring advice.

Regarding the question about the trip North, yes, but further North than we managed last year (the picture was taken last summer). The aim is to get to the Shetlands via the Caledonian canal. If I don't get that far, I shall have bought a lot of charts in vain. (I could have bought two new brand DSC radios for the price of the charts.)

 

[Skylark]

As already suggested, an SWR (standing wave ratio) meter is the best tool for checking integrity of coax and antenna system. Cheap and easy to use, available at Maplins and similar. I see your location is Angelsey. I'll be back in the water 4 weeks from today and I'm also berthed on the island. Send a PM if you would like the use of one.