john_morris_uk
Well-Known Member
Don't know what you think you are watching - there seems to be an air of cordial agreement here.Oooooh, fun.
David pulls up a deckchair and is watching the battle unfold!
xxxxxxx
VHF antenna problems?
- Thread starter Quandary
- Start date
lw395
Well-Known Member
I think the reflected power way of looking at things is a microwave/radar tendency, whereas radio people view things differently. In microwave work, your system tends to be many wavelengths long, so it makes more sense to visualise waves going each way, whereas if you are working at HF, the cables won't often be more than a few degrees long.
I started out in the GHz region, so tend to think in terms of reflection or 'scattering parameters'. Frequencies are too high to measure voltages in a meaningful way.
Marine VHF is near 2 metre wavelength so with a 15m cable, it seems valid to visualise power being refelcted back and forth. But nowadays you could just measure the voltage or look at it on a scope.
In the old days, people used to measure the peaks and troughs in the standing waves, using a sampling detector across the line or a slotted waveguide. The VSWR is the ratio of peak to trough. Nowadays we measure the reflected power. Even most VSWR meters are measuring reflection coefficient and calculating VSWR from it.
Some people seem to spend half their working lives translating between VSWR and S-parameters, but all the test equipment I use tends to work fundamentally in reflection coefficient. When it comes to analysis, reflection coefficient is complex, i.e. it has phase information, whereas VSWR is a mere real, positive number.
OTOH VSWR is strictly a measure of a system, i.e. a source, line, load combination.
Whereas the reflection coefficient is just measuring the line and load. We would then do some sums and see how it interacts with the source, unless we can assume the source is well enough matched to the cable.
A 1.8:1 match is in my terms about an 11 dB return loss, which is quite good except that 6dB of that loss is in the cable (there and back), so the antenna has a ref. co of about 5dB. That's not great, but is probably acceptable. It's equivalent to losing over a quarter of the transmitter power (would you buy an 18W transceiver when you could have 25W ?), but as you say makes negligible difference to the real world. It matters more in what I'm working on at the moment, because the energy that doesn't get radiated goes into melting the transmitter!
As you say, these measurements are just different tools, often to achieve the same end result.
An elaborate way of looking for a broken wire or short! (when you haven't got a time domain reflectometer handy).
john_morris_uk
Well-Known Member
All points noted. We used to sometimes measure the loss in a line by shorting the end and measuring its VSWR at the particular frequency we wanted to use it at - you can plot a graph of loss vs VSWR on a shorted (or open circuit at the other end) line to give you a quick and dirty way of measuring loss.
I was also in microwaves thirty years ago at what was then RRE Malvern. I spent some of my time in a lab measuring the noise sidebands of microwave sources.
I was also in microwaves thirty years ago at what was then RRE Malvern. I spent some of my time in a lab measuring the noise sidebands of microwave sources.
Scotty_Tradewind
Well-Known Member
Can you get a spare aerial to get you by if your problem is up the top?
I have a spare metz aerial in my locker which came with the boat, which is a handy item to have if things fail when you are out.
I have a spare metz aerial in my locker which came with the boat, which is a handy item to have if things fail when you are out.
Quandary
Well-Known Member
Yes
I have been using my emergency antenna taped to the sprayhood frame.
I am really confused now by the debate above and have concluded that it is well above my head and since I don't have a VS??? meter anyway, the only solution for me is to buy a new antenna and try and fit it. The Metz would require a connector to be soldered at the top of the mast while a replacement like for like Vtronix would require the existing cable to be drawn out of the top of the mast with a messenger to allow the new cable to be pulled down through. I think the Metz may be better but I am suspicion of having to fit a connector outdoors at the top of a 15m. mast
I had hoped some one would just say do X or Y in a convincing way, rather than blinding me with apparently disputed science. I suppose I am really too backward to ask for advice on here and there I was, just about to ask for help with my calorifier.
I have been using my emergency antenna taped to the sprayhood frame.
I am really confused now by the debate above and have concluded that it is well above my head and since I don't have a VS??? meter anyway, the only solution for me is to buy a new antenna and try and fit it. The Metz would require a connector to be soldered at the top of the mast while a replacement like for like Vtronix would require the existing cable to be drawn out of the top of the mast with a messenger to allow the new cable to be pulled down through. I think the Metz may be better but I am suspicion of having to fit a connector outdoors at the top of a 15m. mast
I had hoped some one would just say do X or Y in a convincing way, rather than blinding me with apparently disputed science. I suppose I am really too backward to ask for advice on here and there I was, just about to ask for help with my calorifier.
tt65
Well-Known Member
How to test for VSWR
Mine was tested with an ancient Bird thruline meter. VSWR simply cannot be measured without the proper test equipment. In my days as a marine radio technician I used a fancy unit from Marconi Instruments which cost an arm and a leg and a bit more and was matched with another bit of kit which measured deviation. These units were built specifically to test marine VHF equipment.. I would get my friend to test yours but he and his Bird Thruline are in Cardiff, I am in Portsmouth and you are some 500 miles away from us. Sorry.
Just for fun try the following. Disconnect the aerial from the back of the set. Then touch the centre pin of the plug to the centre of the socket without allowing the screening metalwork to touch. (DO NOT TRANSMIT). (In effect you disconnecting the screen of the coax from the set) You may find that you get near normal reception. If so, it is simply confirmation that either the cable, the aerial or any plug/socket is banjaxed.
Personally I would replace aerial and cable and any through deck plug/socket arrangement with new kit. On reflection, if you have a through deck plug socket arrangement as I have, open it all up and check it is still sound with no corrosion or damp.
Hope this helps you and stokes up the debate somewhat.
Mine was tested with an ancient Bird thruline meter. VSWR simply cannot be measured without the proper test equipment. In my days as a marine radio technician I used a fancy unit from Marconi Instruments which cost an arm and a leg and a bit more and was matched with another bit of kit which measured deviation. These units were built specifically to test marine VHF equipment.. I would get my friend to test yours but he and his Bird Thruline are in Cardiff, I am in Portsmouth and you are some 500 miles away from us. Sorry.
Just for fun try the following. Disconnect the aerial from the back of the set. Then touch the centre pin of the plug to the centre of the socket without allowing the screening metalwork to touch. (DO NOT TRANSMIT). (In effect you disconnecting the screen of the coax from the set) You may find that you get near normal reception. If so, it is simply confirmation that either the cable, the aerial or any plug/socket is banjaxed.
Personally I would replace aerial and cable and any through deck plug/socket arrangement with new kit. On reflection, if you have a through deck plug socket arrangement as I have, open it all up and check it is still sound with no corrosion or damp.
Hope this helps you and stokes up the debate somewhat.
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Heckler
Well-Known Member
OK
go up the mast, there is a plastic nut that holds the cable in the arial, undo it and GENTLY pull it out, if it has corroded gentle wiggling and fiddling reqd. You should end up with the cable terminated by a brass knurled thing which unscrews off the cable. If you are lucky it will be fairly clean with water in/on it. Clean and dry it, vaseline it and replace it. If its verdigeed and manky then your choice
Stu
tt65
Well-Known Member
Very High Frequency Calorifier!!!!!!!!!
I have a calorifier too.
The only problem I ever had with my calorifier was when the pump that pumps the engine heated sea water through the calorifier stopped pumping. At first I did not even know that there was a separate pump for the purpose. Any way, no hot water until I cleaned the pump which was gunged up with gunk.
I have a calorifier too.
The only problem I ever had with my calorifier was when the pump that pumps the engine heated sea water through the calorifier stopped pumping. At first I did not even know that there was a separate pump for the purpose. Any way, no hot water until I cleaned the pump which was gunged up with gunk.
Quandary
Well-Known Member
Thanks guys
Thank everyone who tried to help, I appreciate now that this is a technical subject, but I like Stus down to earth response (except the bit about, 'go to the top of the mast' as we are having a lot of horizontal rain here these days) and the useful pm which I got from Salty John; as stated above, there is good information on his website. I am sufficiently encouraged to consider posting my calorifier query.
My plan is to buy a new Metz Antenna and cable from Salty John, fit a connector to the top, cut the existing cable at the mast foot, attach a cord, go up the mast pull it through and fit the bottom connector in the duct in the hollow table leg. I am hoping I canuse the existing V'tronix bracket which extends straight back in line with the mast crane(the Metz bracket fits offset to the side of the mast) and also slip the existing windex over the top.
Even better; I now have three mast climbing volunteers (our international PBO forum crew for West Highland Week)
Thank everyone who tried to help, I appreciate now that this is a technical subject, but I like Stus down to earth response (except the bit about, 'go to the top of the mast' as we are having a lot of horizontal rain here these days) and the useful pm which I got from Salty John; as stated above, there is good information on his website. I am sufficiently encouraged to consider posting my calorifier query.
My plan is to buy a new Metz Antenna and cable from Salty John, fit a connector to the top, cut the existing cable at the mast foot, attach a cord, go up the mast pull it through and fit the bottom connector in the duct in the hollow table leg. I am hoping I canuse the existing V'tronix bracket which extends straight back in line with the mast crane(the Metz bracket fits offset to the side of the mast) and also slip the existing windex over the top.
Even better; I now have three mast climbing volunteers (our international PBO forum crew for West Highland Week)
john_morris_uk
Well-Known Member
I am sure that is a good plan - just remember to put the plug on the cable properly and seal it against moisture. Salty John uses connectors that aren't designed to be waterproof but on the positive side they are cheap and they do work if you keep the wet out!
