VHF antennas - DC grounded better in practice?

[Anonymous]

It seems that most run of the mill VHF antennas are not dc grounded (i.e. they look like an open circuit on a multimeter) and that the better quality higher-spec ones are dc grounded and look as though they are shorted when you measure with a multimeter. Leaving aside the technical specifications (which I have read and understand) does anyone have a view as to whether it is worth paying more in practice (since we are limited by line of sight, and the installation)? Does a slightly lower VSWR really matter? - It looks as though the cheap ones have a VSWR of 2.0:1 /forums/images/graemlins/shocked.gif and about 1.3:1 for the dc grounded.

 

[Boatman]

"Does a slightly lower VSWR really matter?"

Yes it does, no matter what anybody says

It comes down to signal strengh, clarity etc..........

A good earth(grounding is important for any radio)

 

[Oldhand]

Probably depends on the combination of the design of your transmitter and how long you transmit for as reflected power causes heating in the transmitter output stage. A lower VSWR is obviously more friendly to your equipment but I would expect transmitters to be designed for 2:1 VSWR and possibly able to handle 3:1 without distress.

I would suggest the vertical radiation pattern of the antenna relative to the application environment is just as important as the power radiated from the antenna overall.

 

[Billjratt]

The "grounded" Shorted" aerials are probably "J" type. The kind that are usually encased in fiberglass . What you are measuring is the bottom of the "J" which looks like a DC short but is the tuning stub that brings the aerial to resonance at the mid-band frequency.The open circuit ones are probably simple rods with a loading coil in the base. Either way you don't have the choice of earthing or not because the manufacturer has done it for you (correctly) If you have a mobo or cat, (stable platform) choose a long (1.5mtr) "J" if you're a sailor choose a 90cm whip. the polar diagrams are different.

 

[Birdseye]

I've no doubt that in extreme circumstanmces the slight difference in swr could make the difference between being heard or not. Just like the quality of the microphone could, or the level of background noise or the voltage available to the radio, and a hundred other things.

For the remaining 99.9% of the time, I wouldnt think it would matter at all.

personally I would be at least as interested in the physical construction since aerials usually fail at connectors. And I would go for the longer whip since power radiated is related to whip length up to a point.

 

[john_morris_uk]

[ QUOTE ]
"Does a slightly lower VSWR really matter?"

Yes it does, no matter what anybody says

It comes down to signal strengh, clarity etc..........

A good earth(grounding is important for any radio)

[/ QUOTE ] Do the maths. You can argue that it does matter till you are blue in the face, but you can end up chasing fractions of a percentage difference on a scale that is actually logarithmic - in other words a waste of time.

 

[steverow]

The Grounding...or more precisely Short Circuit at DC will indicate usually a 5/8 wavelength antenna.
Open circuit usually indicates a 1/4 wave antenna.
Its all about impedance matching.
A 1/4 wave antenna will present around about 50 or 60 ohms at resonanace so is a reasonable match but the drawback is that the radiation pattern, is about 45 degrees meaning that about 60% of your signal is wasted up into space.
By adding an untuned 1/8wave section on top and using a shunted loading coil to bring the impedance back to the required 50 ohms they "flat top" the signal meaning that much more of it goes out horizontally.
THe loading coil is usually in the base of the antenna and tapped down to dc ground, but at the resonant frequency reads the required impednce.

There are other variants with loading coils the7/8 and full wave antenna but these have different radiation patterns and are not so common.
A loaded 5/8 which will measure a DC short with a multimeter is to be preferred.

Hope this explains a bit.

 

[Anonymous]

[ QUOTE ]
Hope this explains a bit.

[/ QUOTE ]Yes, very useful, thanks. And many thanks to everyone who has replied.

I worked in an aerial design lab many years ago up at X band, mainly, and swotted up to pass my Radio Amateur exams a couple of years ago but it isn't my field, really so I am not the best person to make this judgement.

Given the better matching plus the better (vertical) polar plot we do get a far superior product by paying the extra. However, if we are limited primarily by line of sight communication and if the attenuation with distance is not the issue, then do we actually gain, in practice, with a yacht having a mast of, say, 20m height above sea level? I suppose we are only interested in the range to the coast station?

 

[steverow]

No your'e probably right a 1/4 wave stainless vertical is probably just as good at below horizon comms, bearing in mind the much superior height of the coast station.
Other more effective sticks come into their own for bridge to bridge comms at distance. But always go for a quality antenna if you can, particularly look at the quality of the connectors and cable.

 

[Anonymous]

My interest right now has stemmed from a comms problem - I don't have a VSWR or power meter unfortunately and nobody in Almerimar has a working one, either /forums/images/graemlins/shocked.gif but I found some white corrosion on the SO 239 connector which I have removed and all seems to work functionally.

One obvious advantage of an antenna with a dc short to earth is that it is far easier to test with only a multimeter.

Someone needs to come up with a simple piece of kit with a few LEDs (or cheap meter) that you can put in series with the antenna (R/T > patch lead > test kit > socket > antenna downlead) powered from internal batteries or cigar lighter 12V that will show power radiated in multiples of approx 5W; 5, 10, 15, 20, 25, 30W. Must be possible.

The problem, I suspect, would be that it would be argued that being a life-safety piece of kit it needs to be tested by a qualified technician using calibrated equipment to an agreed test procedure. True, of course, but at present they are tested by non-technical yachtsmen using a multimeter and a quick call to a mate on a H/H! /forums/images/graemlins/smile.gif

Many thanks for your input - it's put my mind at ease.

 

[wooslehunter]

If your antenna has a dc short to ground, you're right you can test that you have a short with a mulimeter. You can test an open circuit too. Neither of these is any use at all if the cable or plugs are o/c or s/c.

Clearly if it measures short when it's supposed to, the cable or connector could be short and you still have a fault.

The reality about VSWR is that John Morris said, as long as it's acceptably low, you'll get little or no difference in performance. Over the working range of a radio, the received signal power will vary over a ratio of millions. The difference between a good & poor VSWR is measured in %.

The main things that make a difference are height & radiation pattern. Radiation pattern is affected by the antenna design, the way it's mounted and oriented.

Generally, a large antenna, mounted high up and vertically on a large ground plane will be best. Antennas designed for marine VHF have a radiation pattern that's like a doughnut. i.e. it's independant of direction (omidirectional) and signal strength will decrease with increasing vertical angle. The longer the antenna, the flatter the doughnut.

There are nice long fibreglass antennas that ideally will give better performance than the shorter stainless types when mounted vertically. This is because the doughnut is flatter. But, yachts rock around a lot more but have higher mounting points. So, you use a shorter 5/8 wave for yachts mounted high up. It's around a meter in length. For power boats, you can use anything up to 6m in length. These are actually phased arrays that give "gain" but have a very flat pattern.

Sorry got carried away.

 

[Billjratt]

I can't believe there isn't a SWR meter in Almerimar. Is Apollo John still there? (he had a flat as well as Apollo, a 50ft yacht, if I remember correctly).

 

[Anonymous]

I don't know anyone of that name, but then I don't know everyone here. Having said that, I know most of the people who know, and I put the word out that I wanted an SWR meter - apparently there are two, but neither has a dummy load and in any case are suspected of being damaged and a very old one in the second hand shop that looks as though it has been used as a door stop for the last 25 years. Again, no dummy load.

I was surprised. Actually, a friend of mine, a ham, does have one and he left the marina the day before (sods law) so I will probably catch up with them in Cartagena and borrow it there - I have managed to get it working acceptably for now.

 

[whipper_snapper]

lots of modern ham sets will have the option of showing swr when transmitting. If they have been 'opened' they will probably be capable of transmitting on marine vhf frequencies.

I would be surprised if there isn't one of those that could be borrowed for the price of a beer.

 

[Anonymous]

That could be possible, thanks for the idea. I found corrosion on the connector, have now cleared it and now the performance has improved to the state where I can reach the entire marina behind large buildings at 1W whereas before I could not manage that on 25W so I think that we are in business for the time being. I have no doubt that we are safe to go to sea and I will either catch up with my ham friend who has an SWR meter or order one from the UK and we can check it out at leisure. I do have an emergency antenna, a handheld, a Mini-M as well as GPS EPIRBS and a marine MF set so I don't feel that I am exposed in any way, given that the present performance seems to be what I would expect.

I am very grateful to everyone for their help.

 

[Birdseye]

[ QUOTE ]
I do have an emergency antenna, a handheld, a Mini-M as well as GPS EPIRBS and a marine MF set so I don't feel that I am exposed in any way

[/ QUOTE ]

Belts? Braces? You've got as much electronics there are a soviet spy trawler! /forums/images/graemlins/grin.gif

 

[Anonymous]

[ QUOTE ]
Belts? Braces? You've got as much electronics there are a soviet spy trawler! /forums/images/graemlins/grin.gif

[/ QUOTE ]Yes /forums/images/graemlins/smile.gif The PO took the boat on the ARC and did a full circumnavigation, he also wanted to stay in touch with his business. I have kept most of it up as it seems a shame to let the standards fall when they are more or less handed to you on a plate like that....I would never dream of buying that lot for pottering around the Med, which is all we are doing at the moment.