VHF Antana Debugging

[Andrew_Trayfoot]

I have an fairly new ICOM VHF that used to transmit fine, but for the last couple of seasons has had very little range.

I have replaced the antenna at the top of the mast and the cable to the foot of the mast with no improvement.

I also have a separate VHF antenna on the pushpin for the AIS and if I plug that into the VHF I get an improvement. Tested in Chichester marian with a hand held in the tender, so not very scientific.

So...

I know you can use a SWR meter to test the signal power output but all the ones I can find online for a reasonable price are for 28mhz CB's not marine 106 mhz VHF's.

So does anyone know if the CB ones will tell me anything useful or where I can get, hire, borrow an Marine Band one?

 

[Andrew_Trayfoot]

I have an fairly new ICOM VHF that used to transmit fine, but for the last couple of seasons has had very little range.

I have replaced the antenna at the top of the mast and the cable to the foot of the mast with no improvement.

I also have a separate VHF antenna on the pushpin for the AIS and if I plug that into the VHF I get an improvement. Tested in Chichester marian with a hand held in the tender, so not very scientific.

So...

I know you can use a SWR meter to test the signal power output but all the ones I can find online for a reasonable price are for 28mhz CB's not marine 106 mhz VHF's.

So does anyone know if the CB ones will tell me anything useful or where I can get, hire, borrow an Marine Band one?

Having had another look prices seem to have come down a bit since I last looked.
Any advice woukd still be greatly appreciated

 

[zoidberg]

It seems your transceiver 'RF power amp' is pushing out enough power into at least one of the feeder cables, so it is likely pushing the same out into the other.... the one which leads to the mast top.

Now... and the cable to the foot of the mast... suggests to me there is a connector thereabouts, one at the tranceiver, and another up high at the feed cable/antenna connection. It is highly likely that your 'signal loss' is down to one or other of those connections, for that's typically where it can be traced to.

I understand it takes considerable experience to use a suitable VSWR meter and create results from the conclusion..... and no marine sparky I've ever met was willing to go to the top of a mast and measure things at the feeder/antenna connection - which is where it should be done. ( and is easily done with a pushpit mounting )

Do try inspecting and remaking any connections you can access. That's cheap and easy. It may result in a satisfactory fix

 

[lw395]

I think VSWR meters lead people astray as often as they help.
I would suggest that antenna cable has a short life. Any damp or corrosion/oxidation in the braid means it's ready for the bin.
Connections need to be made properly, exactly as per the manufacturer's instructions.

Unless the cable is seriously degraded or not the correct stuff, the idea of measuring the VSWR at the top of the mast instead of the bottom is just bollocks. The only time you'll get a misleading answer is when the cable has gone lossy due to oxidation but miraculously maintained a 50ohm impedance. It might be worth considering if the cable has a loss of many dB, like if it's hundreds of metres long at VHF.

So, is the braid shiny?

 

[zoidberg]

Of course it is not suggested that one should shin up the mast to measure the Standing Wave Ratio there - where measurement would be more accurate and less misleading - but the simple 'inspection and remake' suggestion for the connections within reach should help. It is also possible that the coax used by the OP may not be of the best and the shielding on that cable perhaps inadequate. I'd recommend copying the details printed on the relevant cables and asking a specialist such as Salty John.

In my understanding, by far the greatest 2 sources of VHF problems is down to ropey connections and inappropriate coax type..... and it's easy to check those out.

 

[earlybird]

Shouldn't admit this, but I've used a cheap Tandy CB SWR meter on my VHF. It does give indicative relative readings IMO, the actual values will be fairly meaningless I assume.
I kept transmissions down to a few seconds, no harm ensued.

 

[lw395]

Coax condition is much more important than type.
There's a lot of corroded junk out there which does more harm than you'd do by using 75ohm TV coax for instance.
When in doubt, new cable.

It's very easy to check the loss, if you have a Network Analyser. Not many do, as their new cost is high. Mine cost nearly as much as my car. The car was newer!

 

[rszemeti]

Cheap VSWR meters are easily obtained, If it is a cheap one, I suggest you first try it on a friends boat aand get an idea of where the "forward" power needle points on a "good" radio ... and then check yours. It will at least give you an indication if yours is producing an acceptable amount of power. Don't get too hung up on the exact amount ... providing it is producing at least 50% of what a known "good one" produces, it is going to work OK.

 

[lw395]

Shouldn't admit this, but I've used a cheap Tandy CB SWR meter on my VHF. It does give indicative relative readings IMO, the actual values will be fairly meaningless I assume.
I kept transmissions down to a few seconds, no harm ensued.

It's complete pot luck what you get from a 27MHz amateur instrument measuring around 160MHz, some will give a useful approximate indication, some will read utter nonsense.

 

[rszemeti]

It's very easy to check the loss, if you have a Network Analyser. Not many do, as their new cost is high. Mine cost nearly as much as my car. The car was newer!

A network analyzer more than good enough to work out if a yacht aerial is working or not can be had for as little as £50 these days .. check out "nano VNA" on eBay etc. Not exactly Hewlett Packard, but perfectly decent for this sort of work.

 

[lw395]

Cheap VSWR meters are easily obtained, If it is a cheap one, I suggest you first try it on a friends boat aand get an idea of where the "forward" power needle points on a "good" radio ... and then check yours. It will at least give you an indication if yours is producing an acceptable amount of power. Don't get too hung up on the exact amount ... providing it is producing at least 50% of what a known "good one" produces, it is going to work OK.

If you blow up his VHF, you'll be an ex-friend....
If his VHF senses that you've connected a piece of junk to it and backs off the TX power, how will you know what's going on?
Get a VSWR meter calibrated for the 16oMHx band or don't bother. There are reasonably cheap ones out there intended for the amateur 2metre band which stand a chance, there are 27MHz CB ones which are no use to us.

 

[rszemeti]

I don't think I mentioned using a 28mhz one, it goes without saying that the instrument used should be at least labelled for the band specified, unless you have bought something decent such as a Bird model 43, I would always suggest testing it before using it to try and diagnose a fault.

"blow up his VHF" ... you aren't one of these strange guys that thinks reflected power actually comes back down the coax and heats up the transmitter are you?

 

[lw395]

A network analyzer more than good enough to work out if a yacht aerial is working or not can be had for as little as £50 these days .. check out "nano VNA" on eBay etc. Not exactly Hewlett Packard, but perfectly decent for this sort of work.

If you know how to use it.
May be misleading measuring at low power and expecting it to be valid at Tx power.

 

[lw395]

I don't think I mentioned using a 28mhz one, it goes without saying that the instrument used should be at least labelled for the band specified, unless you have bought something decent such as a Bird model 43, I would always suggest testing it before using it to try and diagnose a fault.

"blow up his VHF" ... you aren't one of these strange guys that thinks reflected power actually comes back down the coax and heats up the transmitter are you?

Where else does the reflected power go?
To be fair the wave view of things is not entirely valid in a typical solid state VHF PA, but plenty can be damaged by running into an open, short or other high SWR. Many will have protection from this, resulting in confusing readings.

 

[rszemeti]

Where else does the reflected power go?

Umm ... it doesn't ... you do reallise that forward and reflected power shown on a directional power meter is just a concept and not an actual flow of power, right?

This article will help: Understanding Forward/Reflected Power

 

[Andrew_Trayfoot]

Thanks for the info.
so what does one of these actually tell me?
2.8" Nano VNA MF VHF MF Vector Network Analyzer Antenna Analyzer LCD Digital Kit | eBay

How do I interpret the results?

Mast is down at the moment.

In terms of the setup I have:
2 year old Shakespeare V-TRONIX Hawk VHF Antenna with Wind Indicator. - This was replaced to try and solve the problem..
All Coax is 50ohm
Connector at the top mast if factory fitted

Cable to bottom of the mast is the same age as the antenna (came with it)

So I am guessing the problem is in this lot:

Any way to test the cables?

Cable from mast foot to aft end of port cuddy in the main cabin
Cable from aft end of port cuddy to bulkhead panel (back of the radio).

Cables are connected with barrel connectors and PL259's


With a multi meter the continuity of the non mast cables look ok0 or 1 to 2 ohms

Any suggestions?

 

[rszemeti]

The "nano VNA" was in resonse to someone else, I do not recommend it unless you already know how to use it.

By far the simplest way to check if it is the radio or the aerial is to borrow a radio from someone and try that, if it solves the problem, you have your answer.

If swapping the radio does NOT improe things I would recommend a VHF power/VSWR meter ... I suspect from your description that the antenna and coax is fine, but for some reason either the power is not leaving the radio .. or there is some other problem in the wiring. You should see something like 20+ Watts going forward and a low figure for reflected power.

I would try checking it at the radio to cable point ... and again where the barrel connector is.

Unfortunately, the cable supplied is rather "lossy" ... so even a completely broken aerial will only show something like 5 Watts reflected .. it could be the connection which if I remember is a sort of "push in" contact at the aerial and not a PL259 ... you could try connecting and disconnecting the aerial at the top, if you see no change in reflected power, you know the aerial is not connecting.

I suspect you may find the radio is the problem ...

 

[Skylark]

Where are you based, Andrew? It may be worthwhile asking around to see if you could borrow an SWR meter.

I’m sure that a Google search will find your nearest Amateur Radio club, RSGB affiliated.

I suspect that members would be delighted to help you check the installation using basic test equipment.

 

[lw395]

Thanks for the info.
so what does one of these actually tell me?
2.8" Nano VNA MF VHF MF Vector Network Analyzer Antenna Analyzer LCD Digital Kit | eBay

How do I interpret the results?

Mast is down at the moment.

In terms of the setup I have:
2 year old Shakespeare V-TRONIX Hawk VHF Antenna with Wind Indicator. - This was replaced to try and solve the problem..
All Coax is 50ohm
Connector at the top mast if factory fitted

Cable to bottom of the mast is the same age as the antenna (came with it)

So I am guessing the problem is in this lot:

Any way to test the cables?

Cable from mast foot to aft end of port cuddy in the main cabin
Cable from aft end of port cuddy to bulkhead panel (back of the radio).

Cables are connected with barrel connectors and PL259's


With a multi meter the continuity of the non mast cables look ok0 or 1 to 2 ohms

Any suggestions?

Renew the cable from the mast base to the radio with no joints.

 

[Salty John]

Have you checked the cable from mast base to radio to be sure you have no circuit between centre pin and outer shell on all your connectors?
Most likely causes are corroded cable and badly fitted connectors.