Testing a VHF antennae

[Leisure27]

Following on from a post below:
I am having problems with my VHF and suspect the aerial. Can anyone tell me what instrument I can use to test it, what the test consists of, where to get one and how much it costs?

Thanks, Brendan.

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[MainlySteam]

Yes, you can use an SWR meter. However, 2 proviso's:

- a meter that can be relied upon, and the coaxial patch cord to patch it into your coax, will cost you much more than a replacement ss whip type VHF antenna. You may as well buy a new antenna if you know the coax is ok (it almost certainly will be if the copper braid is still shiny and the connectors are correctly made and measures open circuit between braid and centre conductor and short circuit from end to end through each of the braid and centre conductor when disconnected from both the antenna and the radio)

- despite the readiness of many to advise use of an SWR meter, it is possible to have a satisfactory SWR with a dud antenna/coax.

My suggestion would be that if you needed to ask the question, and if you rely on your VHF for safety, get it checked by a competent marine radio technician. It will likely cost you not much different to buying an SWR meter.

John
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[William_H]

VSWR meter is the way to test it however as pointed out it would be more cost effective to get another antenna you may set it up as an emergency antenna which will prove if your existinjg antenna is dud. Simply you need a coax cable with connector to suit radio long enough to reach outside then some. Cut the outer insulation back about 2 feet and fold the outer copper braid back over itself down the cable. Trim both the remaining centre conductor and the folded back braid to 17.67 inches. This can be strung up or put inside a plastic tube clear of other metal (the folded back braid is part of the antenna and should be insulated and clear of metal)and will work well as an antenna given that not being as high as your mast top it won't work so well otherwise it should be just as good and always available as an emergency. regards will

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[bedouin]

I would have thought that the actually antenna was one of the most reliable parts of the VHF circuit.

If like me you have a deck mounted connector for the coax (a bad idea - I'm replacing mine soon) then I would say that that is the weakest point in the circuit, followed by deterioration of the Coax itself where damp can get in at either end. Taken with the possible problems of connecting a new antenna on to an old and potentially corroded Coax then I would think very carefully before replacing the antenna without renewing the coax too.

I know that sophisticated testing kit is available that can detect accurately where the problem is in circuit, but that is well beyond the reach of an amateur and I doubt if many Marine radio technicians have access to it either.

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[copterdoctor]

I worked my way through the system replacing bits as I went but ended up changing the antennae. If you can hear and not transmit they it is probably the antennae. Try transmitting to a boat very close, you should have some range even with a bad antennae.

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[bruce]

sorry, i don't think you will get a swr meter to give a good reading with a bad cable or antenna unless you have an amp in line after the swr meter. the swr meter reads from it to antenna, making it necessarty to have a good patch cord, short as possible. swr meters are marketed by shakespear antenna and 2 meter ham dealers. a cb swr can be used, but due to difference in freq, a good match will be 3:1 rather than the usual 1:1 at the cb freq. inline meters run about $40.

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[steve_cronin]

My Shakespeare VHF SWR Meter.....

....the proper item..is available on loan for an RNLI donation (receipt available) of £10. (7days use)

Please PM me to make contact.

Steve Cronin



<hr width=100% size=1>The above is, like any other post here, only a personal opinion

 

[MainlySteam]

<<<sorry, i don't think you will get a swr meter to give a good reading with a bad cable or antenna >>>

Without getting into the complications of the theory, perhaps if I give you a simple example. Any 50 ohm coaxial termination of the end of a 50 ohm coaxial transmission line will give a 1:1 SWR and a dummy load used for testing will do that even though the dummy load is an extremely poor antenna. So, there you are, just one example as to how one can get a good SWR with a bad antenna.

Also, SWR meters can be very sensitive to common mode currents on the coaxial cable (and these are more likely when you have an antenna fault) causing them to seriously misread. It may misread towards indicating a correct match at the antenna even though there is a fault. Usually under this circumstance changing the length of the coaxial cable results in a change in the measured SWR which is a fault condition.

Also, if one looks at the relationship between return loss and SWR one will see that as the SWR moves away from 1:1 then the accuracy of the equipment has to be quite high if the accuracy of the SWR measurement is to be protected. The accuracy of cheap SWR meters, especially those oriented to the CB market, is usually very poor. I think you say, and with which I agree, that given the inaccuracy of CB SWR meters in the first place then it is likely that they will be even more inaccurate when used well away from the CB band for which it is sold (there are to my knowledge no CB bands even relatively close to 156 MHz, but not being much into CB myself I stand to be corrected on that).

John

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[bruce]

a broken /bad antenna will not give a 50ohm reading at the radio end. if the loading coil has shorted it will change readout. a dummy load is made up of 50 ohm resistors, enough to handle the watts, but this will show that the radio is transmitting. the 3:1 swr on a cb meter is because of the freq difference, so a 3:1 with vhf is equal to 1:1 in cb/27meg. coax length can alter swr, gen as long as the coax is kept in units of about 18", the wave length at 156+/-, the swr will be good enough. since we are talking about a radio that only goes to the horizion, exact swr match (1:1) isn't that important, but readings of 1:2 or higher will burn out your final power transisters. point is the swr meter is the easiest thing you cad do to check out the radio short of holding a floursent bulb next to the antenna or holding on to it as you key the mike.

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[MainlySteam]

I suggest that you read some professional literature on transmission lines and the matching of them at radio frequencies (the amateur ARRL manuals are also authorative).

What you are saying is totally incorrect and indicates a complete lack of understanding of what you are advising others on. There is not one technically correct statement in the whole of your post and I trust that no one relies on it.

John

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[Strathglass]

I totally agree with your last posting John.

It can be quite frightning when someone sends what superficially an authoritive posting that is in fact complete garbage.

On those forums I suppose one has to sift the wheat from the chaff.

Iain

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[bruce]

sorry, no more advice, but feel free to send me all your failed equipment. KD4NKL

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[Joe_Cole]

John, Iain, or anyone else!

Can anyone point me in the right direction to find out more about making an aerial for a DAB radio? I haven't been able to find one that's suitable for marine use and have been experimenting with a simple dipole; so far the results have been superb and I feel very pleased with myself! However the radio also needs better reception on FM and, if possible on LW (I need the weather forecast!). I suspect that all 3 wavebands on one aerial is hoping for too much, but it may be possible to get better band width to cover FM and DAB. This is when it starts getting a bit beyond me; add to that vertical (DAB) and horizontal (FM) polarisation and I'm out of my depth. I could have two aerials I suppose, but I don't have the faintest idea how to connect them up to the radio. I also quite fancy making a proper whip aerial (more out of interest that anything else)

Any thoughts?

Joe

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[MainlySteam]

Joe

For FM broadcasts the standard in most places is that the transmitted signal is to be vertical or mixed (ie vertical alone or horizontal and vertical together) polarisation. Vertical because of for reception when mobile such as in car and boat FM broadcast receiving stations. So you can use vertically polarised antennas for both DAB and FM services.

In the UK I think, for now anyway, all your DAB is on Band III (around 217 - 230MHz), not L Band (1452-1492 MHz), in which case a vertical half wave diplole is fairly easy to make. But if L Band becomes to be used then it will not work for that.

The standard for FM antennas is 75 ohm (just like TV) but my understanding is that the standard for DAB is for 50 ohm antennas and that there should be a different connector for each antenna on the back of your receiver - but it sounds like that is not so with yours if it is an automobile set you are using.

If you wanted to try an antenna that covered DAB Band III and FM then if you made a vertical dipole for halfway in between, say around 160 MHz then you may find that gives you some results but not as good as a correctly cut antenna for each. By chance, the 160MHz I suggest is about right where marine VHF is so what you could do to check how well such an antenna would work is to try it with a marine VHF whip if you have access to a spare one. Preferably a 5 foot one rather than the shorter 3 foot odd ones but the 3 foot one would give a good idea still. Otherwise you could cut one from wire just to try.

I suspect that it will not work brilliantly but may suffice and if it does then you have a ready made marine quality antenna for AM/DAB plus a spare for marine VHF. We do not have DAB here for me to do a check myself.

Moving on from that, it is my experience that if one wants to make a good antenna for marine/outside use at VHF frequencies then it is usually easier and often no more expensive to buy a commercial one by the time one has had a failure or two and the fact that a home built one may not last as long. I have found that reasonable non marine antennas last ok in the marine environment and are much less expensive (for example, we use an ordinary car cellphone antenna and an AM/FM fibreglass antenna designed for long distance car/truck use). One just has to make sure water cannot get in which means that the antenna part has to be encapsulated/solid and the base able to mount up to a deck or something with no corrodible metals in its construction.

If you want to cut and try some wire to make up a trial antenna for around 160 MHz but do not know how to work out the lengths just PM me. Similarly, if you want to build a weatherproof one for DAB alone and do not know how to go about that. Also, if you only have one input on your receiver but find that you need two antennas, let me know because maybe there is a cheap solution to try for that.

In the meantime someone may come up with a proven answer. In any event I would be interested to know how you get on.

John


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[Bergman]

Don't beat around the bush John tell him straight.

Rather worrying that he managed to get an amateur license. It must be as bad in the US as in the UK.

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[MainlySteam]

I was so disbelieving about the amateur callsign that I checked up on it - he does indeed have a technician's class amateur licence.

John

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[Oldhand]

Reference antenna element lenghts for 160Mhz, the wavelength is 1.875m. thus 1/4 wave for each of the 2 diploe elements is 0.469m. Multiply this by say 0.92 for end effects gives an element lenght of 0.431m (rounded to 3 decimal places, i.e. 43.1cm).

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[bruce]

the only thing i can say is that there is a break down in conversing, and yes i am real, been for many years and my home made antennas in 2 meter have ranged over 150 miles with a handheld 5 watts. am no longer active as i have had my fun with the radios, talked around the world and have lost interest.

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[Joe_Cole]

John,

Brilliant!

I've tried using my spare VHF aerial (a short one), but the results were disappointing. I suspect that, as most places we moor up are in protected harbours, the signal strength just doesn't get down into the valleys.

The aerials I've been playing around with have been half wave dipoles, and the results have been most encouraging both on the boat and my set at home. I've made them with stainless steel welding wire, supported inside fibreglass tubes (well, fibreglass tent poles cut down to size!). I'll try one at the 160MHz frequency but I think that it will be too much of a compromise.

You are right about my sets having a single aerial socket, so I would appreciate it if you would let me know how to feed two aerials into the one socket.

Incidental, as you don't have DAB in your part of the world, it really is good. We live out in the sticks and even with a darned great aerial on the roof we've never had good FM reception where we live; we've only ever been able to receive a couple of stations moderately well. DAB has changed all that, and it really is a pleasure to have high quality reception from nearly 50 stations. I'm working on getting the same on the boat!

Many , many thanks for your helpful reply.

Joe

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[ean_p]

Gettin a little lost now....so for the intellectually challanged amongst us, myself most definetly included and in a fairly simplistic language, what are the best checks to perform on the ariel/ cable assembly assuming no radio technician is available. And is there a relationship between the cable length , the power output and the reading on a SWR meter? This last I ask as the instructions on Glomex ariels suggest that if performance is poor then shorten cable (which is supplied by Glomex) by 12"....?

a poor seeker after truth!

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