Back Stay Aerial

[milfordman]

I am at present planning to replace my standing rigging. I have recently had some problems getting good reception on my VHF (entertanment radio). If I have fitted 2 insulators in the back stay I could use that as an aerial. Would this suit the VHF frequencies, or is it only suited to SW bands.

 

[MM5AHO]

A backstay antenna would be nearly the length of the stay, probably about 10m or more.

An antenna is oten best when its about a wavelength or even a half wavelength.
The wavelength of the VHF (entertainment) band, of 88-108MHz is about 3m. The best antenna would be 1.5m long or so. Your backstay is too long. Its good for the shortwave frequencies, and in some cases too short for them.
For VHF antenna height is very important for distant reception. Possibly best idea is get a high whip antenna. Mast top might be good if space allows, but it would then be close to your VHF Marine antenna, which is a transmitter antenna, and if you were to be receiving on your FM receiver while transmitting on marine band, you might damage something with the 2 antennae so close .
Whip at spreaders? On pushpit?

 

[milfordman]

Thanks MMSAHO,

I tried disconnecting the aerial connection to my VHF radio and touched it to the aerial on FM receiver, the result was, no improvement. Am I doing something wrong?

 

[milfordman]

Sorry I forgot to say the antenna I connected to was mast head mounted, but much less than 1.5m, I am guessing but I think it is about 1/2 a metre.

 

[MM5AHO]

Your marine antenna on the mast head, if connected temp to your FM receiver should show an improvement on weak stations. If not, something else is wrong. The difference in length between your Marine radio and your FM doesn't matter too much in a receiver.

 

[William_H]

The idea of using your backstay as a VHF FM entertainment antenna is not good. Especially considering the cost of insulators for the backstay.

As suggested the VHF marine com may be a good substitute. However some VHF marine ant use a tuned circuit inside and present a short circuit to DC and also to any frequency away from the VHF com band. This type may not work well on FM entertainment band.
However the type that are just a steel rod about 18 inches long and show no connection outer to inner of the cable with a multimeter should be good for FM reception.
However I think you should try another radio if reception is that bad.

I think the ideal antenna for FM receiver will be a VHF marine com antenna as described (steel rod) mounted on the pushpit. Make sure you have connectors that will enable you to use this as an emergency VHF com antenna should the mast top antenna fail or the mast fall down.

If you want to make your own antenna PM me for details. It is essentially coax cable inside a plastic conduit tube with cable and connector to the radio. Mounted on the pushpit I use mine for primary VHF com and it works well but should be OK for FM reception. olewill

 

[Billjratt]

Compare the reception in your car as you drive to the marina. Is Millford crap? Your backstay antenna is ideally suited to HF / SSB type of operation (which may come later) or navtex via a small capacitor. If you can hear BBCx in the car but not on the boat a few meters away, then there is a problem. Over the years, for vhf reception I have used the s/s/ hoops for the sprayhood to good effect.

 

[misterg]

Hello,

Picking up the information in your previous question, you have a steel boat, and a portable radio without the facility to connect an external FM aerial, yes?

(The connection for an external aerial is for SW, only, where you need a *long* aerial to pick up weak signals.)

I assume that the radio works OK on deck?.

The problem would seem to be that the steel hull is stopping the signal reaching the radio's built-in aerial, so you need to know how to connect an external FM aerial to your radio so you could then use a VHF splitter, separate 'car' aerial or w.h.y.

The most obvious route is to change your radio for one with an external VHF aerial ('Car' type, maybe?).

Otherwise, with apologies to the radio purists, it is probably worth experimenting by connecting a length of insulated wire (1-2m) to the telescopic aerial and running it up through the deck. By all means try connecting it to your insulated back-stay, or whatever. Nothing scientific, but if it works, leave it there !

Andy

 

[wotayottie]

You would be able to receive using the backstay, though probably not well. You would have problems transmitting since the length of the aerial needs to be realted to the wavelength of your signal ( about 2 metres) or much of the power transmitted doesnt leave the aerial and gets reflected back into the transmitter. Could even fry your transmitter - or cause it to shut down if it was modern kit.

If your radio is working but poorly, its a pound to a pinch of pig sh*t that the aerial is phut. You need a new proper marine vhf aerial that will match the output impedance of your transceiver - 50 ohms.

 

[john_morris_uk]

[ QUOTE ]
You would be able to receive using the backstay, though probably not well. You would have problems transmitting since the length of the aerial needs to be realted to the wavelength of your signal ( about 2 metres) or much of the power transmitted doesnt leave the aerial and gets reflected back into the transmitter. Could even fry your transmitter - or cause it to shut down if it was modern kit.

If your radio is working but poorly, its a pound to a pinch of pig sh*t that the aerial is phut. You need a new proper marine vhf aerial that will match the output impedance of your transceiver - 50 ohms.

[/ QUOTE ]

With respect, the original post was asking about using the backstay for receiving on his domestic FM receiver.

Also with respect, how come commercial stations use antennas that are multiples of the wavelength of the frequency used?

The only bits I will agree with in your post are that his antenna is possibly shot and that the critical bit about antennas for Marine VHF trancievers is that they should be 50 ohms.

I don't want to get into an argument about power being 'reflected' as it just isn't true. We can go into the higher maths of transmission lines if you want, but the reality is that power is either tranferred or its not.

Voltage Standing Wave Relection Meters tell you something about the impedence mismatch - which tells you how efficient the power transfer is.

 

[Oldhand]

I have found an insulated long wire backstay antenna to be extremely poor at receiving VHF radio signals, definitely not recommended.

 

[milfordman]

Thanks to all for your comments. I am going down to the boat in a few days, and will try out your suggestions.

I suspect you may be right about my aerial, just recently I have sometimes had poor reception. Although during last summer I did a couple of radio checks with the coast guard and they said my signal was OK.

Is there a test that I could do to test the aerial, I have the usual digital multi-meter.

 

[wotayottie]

You are quite right - he was asking about reception only so my comments about half wavelengths are largely irrelevant. He's likely to get away with an odd length of wire in the cabin unless the boat is steel.

I'll turn down your kind offer of the advanced maths - its 40 years since I did my degree so I'm not interested in technical willy waving and probably not up to it any longer. But you do a disservice to others reading if you leave them with the impression that a multiple wavelength piece of backstay can be easily used with solid state output stage marine vhf transceiver by someone non technical - fixed frequency commercial transmission aerials are irrelevant in this context. Nor is it worthwhile debating whether the power leaves the radio and is reflected back or is blocked by an impedance mismatch - either way the risk is of overheating the tranmitter output stages.

So I'll stick to my simple (if technically incorrect) description which I hope is clear enough to help people. Make sure your VHF transmission aerial is tuned to the marine vhf frequencies, or power reflected back when you try to transmit can shut down your radio or even damage it. Never press the tranmit button without an aerial connected and never try just connecting an odd piece of wire whether backstay or coathanger.

In short, just use a commercially made tuned marine VHF aerial connected to your transceiver and nothing else

 

[john_morris_uk]

[ QUOTE ]
Thanks to all for your comments. I am going down to the boat in a few days, and will try out your suggestions.

I suspect you may be right about my aerial, just recently I have sometimes had poor reception. Although during last summer I did a couple of radio checks with the coast guard and they said my signal was OK.

Is there a test that I could do to test the aerial, I have the usual digital multi-meter.

[/ QUOTE ]You comments regarding doing tests with the coastguard concern me. You mentioned 'VHF entertainment' radio in your original post. Which radio antenna are we talking about?

If you let me know which antenna, I will talk about tests, but the tests you are able to do are not infallible.

 

[milfordman]

Sorry for the confusion. My original enquiry was to improve the reception on my FM receiver radio from inside my steel boat. I had tried disconnecting the aerial connection to my Marine VHF transceiver, and just touching the centre conductor to the telescopic aerial that is on the top of the FM receiver radio. I had assumed that this would simulate having a aerial splitter that could feed both the Marine VHF and the FM receiver simultaneously. As I said before this did not improve the reception of the FM radio. The same day the VHF radio was receiving clearly on the port operations channel.

 

[john_morris_uk]

[ QUOTE ]
You are quite right - he was asking about reception only so my comments about half wavelengths are largely irrelevant. He's likely to get away with an odd length of wire in the cabin unless the boat is steel.

I'll turn down your kind offer of the advanced maths - its 40 years since I did my degree so I'm not interested in technical willy waving and probably not up to it any longer. But you do a disservice to others reading if you leave them with the impression that a multiple wavelength piece of backstay can be easily used with solid state output stage marine vhf transceiver by someone non technical - fixed frequency commercial transmission aerials are irrelevant in this context. Nor is it worthwhile debating whether the power leaves the radio and is reflected back or is blocked by an impedance mismatch - either way the risk is of overheating the tranmitter output stages.

So I'll stick to my simple (if technically incorrect) description which I hope is clear enough to help people. Make sure your VHF transmission aerial is tuned to the marine vhf frequencies, or power reflected back when you try to transmit can shut down your radio or even damage it. Never press the tranmit button without an aerial connected and never try just connecting an odd piece of wire whether backstay or coathanger.

In short, just use a commercially made tuned marine VHF aerial connected to your transceiver and nothing else

[/ QUOTE ]I hope that nothing I said implied that you can connect your VHF tranceiver to your backstay. I agree that it would almost certainly not be a wise move.

I was trying to correct a couple of myths that get repeated.

Firstly, antennas CAN be multiples of a wavelength long. They do not all have to be half-waves or quarter waves.

Secondly its incorrect to talk of power being reflected back. It would be better to talk about 'mismatches'.

I agree that you should only connect your VHF to a correctly matched (invariably 50 ohms impedence) antenna and coaxial cable. In the context of a domestic FM receiver it probably won't matter one way or the otherwhat you try as an antenna, and you will do no harm by trying anything (short of connecting your VHF radio to it and pressing transmit!!).

 

[john_morris_uk]

[ QUOTE ]
Sorry for the confusion. My original enquiry was to improve the reception on my FM receiver radio from inside my steel boat. I had tried disconnecting the aerial connection to my Marine VHF transceiver, and just touching the centre conductor to the telescopic aerial that is on the top of the FM receiver radio. I had assumed that this would simulate having a aerial splitter that could feed both the Marine VHF and the FM receiver simultaneously. As I said before this did not improve the reception of the FM radio. The same day the VHF radio was receiving clearly on the port operations channel.

[/ QUOTE ]

I am afraid that it wouldn't simulate having a 'splitter' trying it out the way you describe. I suggest you try borrowing a 'car radio antenna' and plugging it into a designated socket on your FM domestic radio. You might find a small car radio antenna mounted low down on the boat (e.g. the pushpit) would be perfectly adequate for FM radio reception.