VHF Antenna testing?

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Inselaffe

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Hello

I've just fitted my new Navman 7100 DCS VHF, and not altogether surprisingly to me, it dont work!

I can't pick up anything. I have a new Gomex antenna about which I'm a bit confused.

How do you test if you've connected an antenna properly, or if the antenna is ok?

Should there be a circuit between the outer and inner connectors at the set end of the cable, but only through the antenna - in other words, no connectivity if I remove the antenna?

What I mean is, how does the outer shielding cable connect - is there a connection through the antenna (but only through the antenna) between inner wire and shielding outer wire?

I'm confused because initialy someone told me there should be no connection at all between outer and inner wires. After fitting and making multimeter measurements there was a connection, so I removed the antenna and there was a circuit between the two connections on the antenna itself so I thought new antenna was Kaput. Got another one and it was the same, so presumed my advisor was wrong, now the set dont work, I am not so sure.

I also have worries about the antenna, as I bought it in Portugal, and on the plane home I had to put it in as bagage at last moment with no packaging, as the idiots at secu-tity said I could use it as a weapon, and couldn't bring it on as hand luggage - palhaços! I mean where will it stop, the little old lady in front of me could have used her handbag strap as a garrote. So maybe it got a bash by those gentle baggage handlers.

Sorry If i have rambled on, I am a bit unsettled at the thought that if we lose today my Tuetonic wife, amongst many others here in Germany, will make my life hell /forums/images/graemlins/smile.gif
 
Can't do anything about potential humiliation by German family, but there should be no conductivity between the centre pin and the outer body of the PL259 connector on the end of your antenna cable. If there is you can damage the radio.
If you stick a piece of coathanger type wire, about 500mm long, into the antenna socket on the set, being careful not to let it touch the outside of the socket, you should pick up something. If you can, but can't with the antenna, your antenna is knackered. If you can't pick up anything with either, your radio could be at fault.
 
Most VHF but not all antenna will be a short circuit with a multimeter. Coax cable should always have no conection when not connected to a antenna. PL259 conectors should soldered. Do not transmit with out a antenna. Any bit of wire will recive a strong singnal. A handheld next to the radio will be recived with nuthing in the antenna scoket.
 
[ QUOTE ]
Can't do anything about potential humiliation by German family, but there should be no conductivity between the centre pin and the outer body of the PL259 connector on the end of your antenna cable. If there is you can damage the radio.

[/ QUOTE ] I am afraid that you are WRONG! Most boat VHF antennas will show a short circuit to DC (ie on a multimeter) at the plug end when the antenna is connected at the other end. If there were a short when the antenna was NOT connected, then there is a fault.

(The reason there is a DC short is that the centre of the coax is tapped part way up the matching coil in the base of the antenna. This ensures that the impedence match is 50 ohms and correct.)
 
If the cable is disconnected from the antenna and the set you should only get continuity from screen - screen and inner - inner. There should be no connection between inner and screen.

If the antenna is connected, a dc test is meaningless since as already said there is highly likely to be a dc short circuit from inner to ground within the antenna. This normal.

The simplest way to test an antenna is to use a VSWR bridge. Be careful with CB bridges. They are often no good at marine VHF frequencies. The bridge effectively measures the ratio between the power that goes up the antenna lead and the power that returns. If there's ashort at the antenna end, somewhere in the cable or at the radio end, all of the power will be reflected back. There's no way using a VSWR bridge to tell if there's a short or an open connection or where it is in the cable/antenna. You need far more complex kit than that to tell.
 
I had a faulty PL259 socket which was short circuited through the "insulation". Bought a new socket from maplin and it works fine.
 
[ QUOTE ]
Can't do anything about potential humiliation by German family, but there should be no conductivity between the centre pin and the outer body of the PL259 connector on the end of your antenna cable. If there is you can damage the radio.

[/ QUOTE ] Excuse me - you don't say anything about the antenaa not being on the end.

Of course there shouldn't be a short between the outer and inner conductors of just the coaxial cable, but that is not what you said. In fact the implication is that the antenna is connected - 'on the end of your antenna cable' implies to me that the antenna is there.

Anyway excuse me for shouting, and I am not picking a fight over semantics, so if you feel agrieved at my post I will go away.
 
oh, am I happy tonight, and you can say what you like about the Germans, but their beer is the best /forums/images/graemlins/smile.gif

I have the antenna conected, and there is continuity between outer and inner. I have traced it so that there is no connectivity because of the cable or connectores, the connection is through the antenna.

I think that if i disconect antenna there will be no connectivity.

but do these tests have any relevance? why isnt my set receiving?
 
You are VERY wrong.
The antenna should measure 50 Ohm impedance not resistance.
Impedance is opposition to AC current flow at the resonant frequency...not DC resistance.
Your multimeter cannot measure this.
VHF colinear antennae (that is any antenna more than a quarter wavelength long) have to matched. This is usally done by a tapped coil in the base of the antenna, and WILL show a short circuit at DC.
The antenna is not faulty.

Steve.
 
Obviously antenna may or may not have a DC short centre to ground depending on design. The idea of using a piece of wire to check for received signals is good. If all else fails and you want a design for a simple emergency VHF antenna send me a PM with email address. olewill
 
Can I play?

I think Js48 is correct. The questions were rather ambiguous but:

"Should there be a circuit between the outer and inner connectors at the set end of the cable, but only through the antenna - in other words, no connectivity if I remove the antenna?"

Is pretty clear. The answer is, there should be NO connectivity across the cable without the antenna attached.

However the next para asks a totally different question:

"What I mean is, how does the outer shielding cable connect - is there a connection through the antenna (but only through the antenna) between inner wire and shielding outer wire?"

The answer to that is maybe!

So may I suggest we don't jump to criticise a poster who was doing his best with an ambiguous question?
 
I am testing just with a multimeter at the set end of the cable.
I have a closed circuit when the antenna is connected to the cable.
When I remove the antenna there is an open circuit.

I would think I would be very unlucky to have a brand new set that doesnt work, ok its not impossible, but my first assumption is that it is either the antenna or the cable, but how do you test this assumption?

I have new cable up the mast, but haven't replaced the existing cable from set to through deck connector. I will probably replace the old part of the cable and also the connector now, but would like to be able to test to see if it is whats wrong before I go to this trouble.

Similarly, perhaps my antenna is kaput, but before I buy another new one, it would be nice to be able to test it.

But re-reading the posts properly /forums/images/graemlins/blush.gif, it seems I need more complex equipment to do this?
 
Im very suprised your not picking up anything?That is if your aboard your boat.should you be worried about the radio just put any longish (3 feet) wire in the anntena socket and listen for say a forcast or coastal transmision.

I have been told a metal coat hanger will give a good swr!ok ok!!

To try your TX dont do as i would!But ask your local radio repair shop how long a half or quater wave diapole should be cut 2 wires to length attach one to the core the other to screen and transmit to another boat.First check your connector is properly connected to the coax.

Check its not shorting and that the pin shows a reading at the end core then the screen shows a reading to the plug body,Ive never come accros a broken VHF there so well built.
 
I believe with a VHF antennae I had (a 1m wip type), a DC resistance of 1 Ohm was to be expected across the connector on the antennae. If your antennae is already connected to its cable that resistance will be higher because of the DC resistance of the cable, difficult to measure accurately with budget meters and a long cable run.

On that type of antennae which I dismembered (could not repair it) there was a coil at the bottom of the antennae which was connected to both the screen and the central pin of the connector. I believe the coil may act as some sort of ground plain.

As others have said, the IMPEDANCE at radio frequency will be much higher.

Have you programmed in your MMSI number? Can't think why that should stop the set receiving on VHF but maybe??

David
 
[ QUOTE ]
I am testing just with a multimeter at the set end of the cable.
I have a closed circuit when the antenna is connected to the cable.
When I remove the antenna there is an open circuit.


[/ QUOTE ]

That is exacly what you expect to see.

To determine if you have a problem with antenna or set - fit a different antenna! eg your emergency antenna. If you truly hear nothing when you or someone in a nearby boat transmits with a handheld, then you probably have a poblem with the set (you have checked the volume and squelch settings?) Do you hear static when you wind back the squelch ? Is this silenced when someone presses the PTT switch on a nearby radio?

*If you don't have an emergency antenna, you can make a simple dipole out of 2 bits of wire (two bits of wire each 0.46m long, connect one to the inner and the other to the outer conducter. Tape them to a eg a bit of garden cane to keep them straight (with the feed in the middle). That is a perfectly good antenna (and dc open circuit!).
 
Why don't you read the original question and my answer?
"Should there be a circuit between the outer and inner connectors at the set end of the cable, but only through the antenna - in other words, no connectivity if I remove the antenna?"
Answer: "Can't do anything about potential humiliation by German family, but there should be no conductivity between the centre pin and the outer body of the PL259 connector on the end of your antenna cable. If there is you can damage the radio."
In other words - yes, you are correct, under the circumstances you describe.

Contributing to this forum would be a more pleasant experience if there was less likelihood of people climbing in with ill-mannered comments. We might encourage more participation if the normal rules of polite discussion prevailed.
 
Thanks very much, and sorry if my Q's weren't clear enough and have caused unrest.

So the connectivity that I have is normal.
I wanted to make sure it was before trying to sort it out (especially because the guy here in Deutschland told me initially that it wasnt normal)

To be honest I just fitted the thing, turned it on and waited for the hourly forecast and heard nothing. It was a nice day so I went sailing for the day, left it on (with volume up and squelch just up enough to stop static) and didnt hear anything except one time when static cut in.

So I havent yet tried any tests with nearby radios, wire, etc. but now I will be able to do that this weekend.

Benbow, when you describe the simple emergency antenna, (re 'Tape them to a eg a bit of garden cane to keep them straight (with the feed in the middle)' ) you mean the feed should be at the middle of the two wires, ie the cable from the set joins the two wires on the stick in the middle of their length. (hope you know what I mean - I need to draw a diagram!)
 
I found these pictures on the web.

You can bodge much rougher than this!

The white cable is the coax from your set.

halfwave.jpg


PCB.jpg
 
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