VHF antenna - strange effect!

[sailorbaz]

I have a GME 2m whip antenna fixed to the deck at the rear quarter just inside the pushpit. It has a small coil in the middle covered by white plastic. It's been working ok for logging on and off with the local Marine Rescue in Jervis Bay where we sail and also picking up local traffic. Someone climbing on board grabbed it and bent it at the coil thingy. It still worked ok but looked a bit wonky, so I found a short length of white powder coated ally tube in my junk which just fitted over the bend - the taper stopping it from sliding further down. I put self amalgamating tape top and bottom and the finished job looks quite good.
However it's made a huge difference to reception. I have had to to turn the volume down to less than half what it was before and I am now picking up transmissions over 100 km away!
Does anyone know what's going on here?
Sailorbaz

 

[rosssavage]

So the new tube is over the aerial whip or the coil?

Either way, congratulations, you've just invented an iAerial Patent the tube and sell for millions...

 

[lw395]

It's probably broken inside where it bent, and the tube holding it straight fixes that.
Whether it's a reliable fix is another question.
It's possible there are more esoteric effects going on, like the tube somehow compensating for the effect of the pushpit.
But the volume of the audio is not related to signal strength on an FM signal, so perhaps you just have an intermittently faulty radio.

Either way, it sounds like an installation not to be trusted.

Does pushpit imply a sailing boat?
If so a 2m whip is inappropriate, as its pattern is too directional in elevation, so if the boat heels, the antenna won't be seeing the horizon. Different on a multihull of course.

 

[nigelmercier]

But the volume of the audio is not related to signal strength on an FM signal...

Beat me to it. When the OP says mentions a coil, I suspect he means a spring, so it is possible that the antenna was not tuned properly (or is not for the marine band) and is now getting better reception having been shortened.

 

[MM5AHO]

If the thingy in the centre was really a coil, it will be some sort of loading coil. This will be part of the antenna tuning, and it tunes by its "inductance" (an electrical property).
The inductance of a coil can be altered by changing the length, the number of windings, and by adding conductors (particularly iron ones, but not only) near the coil.
By now having the coil inside an aluminium tube section, you will have altered the inductance of the coil and so the tuning of the antenna.

As mentioned above on an FM signal, it doesn't get louder by being a stronger signal really, just more clear. So if the volume is now louder, thats something else affecting things.

If you are hearing more distant stations than before, it could be that your antenna wasn't in tune for the frequencies in use, and by changing the tuning (accidently) you might have improved it.

But further VHF signals from further away are not consistent. The distance a signal can be received from depends on may things and in particular certain conditions in the air (troposphere). A likely time to get enhanced reception is at the end of a period of high atmospheric pressure.

 

[haydude]

Antennas must be accurately tuned to frequency and wavelength.

An antenna to work must have two elements for the positive and negative poles to complete the interactive radiation function.

The simplest to understand and most effective antennas are dipoles where one end is connected to positive and the second end to the negative (antenna wire). The simplest dipole is built using two single core wires of the exact same length and extended in opposite directions. These are "balanced" antennae simple to build (just one wave length) and to tune. They resonate at the working frequency because two waves are created on the opposite poles that are "in tune" with each other and thus "resonate" radiating air waves.

Dipoles unfortunately are impractical on a boat because they are large and directional.

The most common marine VHF antennae are vertical monopoles (the type you have). Vertical monopoles are unbalanced antennae, simply because they do not have the second negative pole. Their tuning depends on their length and the interaction with the ground which is where the negative pole is connected.

A tuned antenna normally presents a 50 Ohm impedance to the transmitter. This is the resistance presented to the transmitter when a modulated wave (alternate voltage signal) is transmitted through the antenna at the frequency to which the antenna is tuned.

At the tuned frequency the antenna is capable of radiating (transforming) all the energy received from the transmitter into air waves.

If the antenna is altered, then also its impedance at the working frequency is altered. In this case the antenna is no longer capable of transforming the high voltage/current wave signal into air waves, and reflects some of this energy back to the transmitter. The energy reflected back to the transmitter overheats the transistors, these eventually can fry when they become too hot.

The ratio between the amount of energy transformed by the antenna and the energy transmitted back (Standing Wave) is called Standing Wave Ratio.

If the sleeve that you have fit is electrically isolated from the antenna, then it works either as a lens or a reflector, most likely attenuating the signal in one direction and improving it from others. Practically it would work similarly to a lens in front of your eyes. A lens' capability to focus a signal depends on its distance from the radiating/receiving element.

If the sleeve is electrically connected then you have altered the tune of the antenna. In this case I recommend to check the Signal Wave Ratio (using a fairly inexpensive SWR meter that you can find on eBay) If the SWR is too high you risk frying the final transistors of your VHF during transmission.

 

[sailorbaz]

Many thanks for your replies. It's on a 25 foot sailing boat and was on it when I bought it. I intend changing it for a mast head aerial next time the mast comes down.
The tube I've put on is insulated from the aerial. The distant station I'm picking up is Port Kembla which in a straight line is over land behind hills and I was getting reception when heeled last Sunday - it was a really good sailing day.
Sailorbaz

 

[cornell_b]

in didn't notice anybody mentioning attaching a swr meter to check the transmitting ability of the antenna.

 

[cornell_b]

antenna P.S.

sailor mentioned his antenna was a '2 meter' antenna, which is a different freq than marine vhf. while close, the antenna should be tuned to one or the other for best power output. just another thought from a 'ham' operator.

 

[William_H]

VHF antenna

The story by Haydude isdescriptive and correct except that of course a balanced antenna is not one wavelength long but 1/2 wavelength.
Your antenna sounds like a common type where the coils and length make up 3/4 wavelength which as said has gain by focusing the signal in a horizontal plane. A shorter 1/4 wavelength rod 17 inches long will have less gain but will be usable at heel angles. The shorter antenna however will also be lower so less effective on that score.
This type of antenna as said must have a ground plane. This can be a large mass of conductive metal mast or car body which is connected to the screen of the coax cable. Some antennae like a UHF Cb antenna will have a tube of metal as part of the base which is 1/4 wavelength long connected to the coax at the top but in this case insulated at the bottom.
Other types of antenna typically in a long white f/g tube have both parts of the antenna internally so are suitable for fibreglass boat with no ground plane or earth needed.

The aluminium tube you splinted the coil with would normally detune the antenna however if your antenna does not have a ground plane then the tube may have brought it closer to correct tuning from badly out of tune. Your antenna may have a ground plane consisting of wires connected I guess under the deck and (hopefully) radiating out from the base of the antenna and connected to the shield.
Unfortunately the proximity of the pulpit upright will also detune the antenna if it is close.
A good alternative would be to mount the antenna on the top of the pulpit post using the pulpit as a ground plane. Or mount it onto a metal tube clamped to the pulpit upright. The higher the better. (and not used as a hand hold)
I have always used a pulpit mounted VHF antenna rather than a mast top antenna. Mainly because it will work if mast falls down but also because it is easier for maintenance if wires break etc.
I would suggest you mend the antenna with a splint of plastic or fibreglass rather than a conductive metal then proceed to get the antenna working correctly.
Certainly antenna design is a bit of a dark art. good luck olewill

 

[rosssavage]

in didn't notice anybody mentioning attaching a swr meter to check the transmitting ability of the antenna.

How about haydude's response two replies above your post!!

In this case I recommend to check the Signal Wave Ratio (using a fairly inexpensive SWR meter that you can find on eBay) If the SWR is too high you risk frying the final transistors of your VHF during transmission.