VHF Aerial Experts...

misterg

misterg

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How well do you think this would work as a VHF aerial?

Do you think it would give 70 mile range? 20 ?

2672235400_ea18c1ab14_o_d.jpg


(The gubbins at the other end to the plug is a 50 ohm load.)

Andy
 
To put your 70 miles into perspective, VHF range is line of sight: D1+D2 = 1.23 x root H1+H2.
D1 = distance of transmitter to horizon, D2 is distance of receiver to horizon. H1 & H2 = height of transmitter and height of receiver.
If the transmitting antenna was 1000 ft high and the receiver 100 ft high they could communicate at 51 nautical miles. Atmospheric conditions will sometimes allow VHF communication over bizarre distances by bending the signal, but not something you would rely on.
I don't know what that piece of , err, technology is that you have there but it isn't a VHF antenna.
It seems to be a cable with a PL259 badly fitted at one end and a complex blob of resistors at the other end. What is it from?
 
As has been said the thing is a dummy load used for testing radios.
The whole design point of the thing is that it is a lousy inefficiant aerial and will only transmit a very short distance in order not to cause problems to other users.
Singing to yourself on CH16 with an audience of testy port ops people is a bad thing and the day will def get worse when you drop key.
 
[ QUOTE ]
that there is a dummy load, should get out about 20 feet with that lol

[/ QUOTE ]

Yep, that was my sentiment, too.

I'd brought the DSC VHF home to sort out a problem with the GPS interface (long story), but while I had it rigged up, I thought I'd connect up a dummy load and see what *actually* happened when I pressed the Big Red Button. (I had ideas of getting SWMBO and the kids to have a practice, too.)

So I did it. (I selected "Piracy" as the disaster of choice.)

The set did its stuff, and sat there "boinging" away with the message "Distress Call To All Ships Channel 16: Waiting for Acknowlegement"

I was somewhat puzzled when the boinging stopped, and the display changed to "Distress Call: Acknowlegement received from MMSI @@@@@@@@"

"That's odd", I thought.

I looked away, and looked back, and it was the same.

"Very odd".

I was positively surprised when the VHF burst into life "Vessel MMSI#######, this is Holyhead Coastguard"

I caught the last few numbers of the MMSI number, and like a pedantic three year-old in slow motion checked them off against my MMSI number.

OH SH!T !!!

The coastguard called again, and images of hovering helicopters, and huge bow waves of lifeboats launching off slipways flood my brain.

Buggerbuggerbuggerbuggerbugger.......

I've got the coastguard number in my mobile, so call them - number not recognised (I'd programmed it in wrong, but the mistake wasn't immediately apparent).

Buggerbuggerbuggerbuggerbugger.......

By the time I've found the yellow pages, or fired the PC up, the helicopters and lifeboats will be on their way.

Buggerbuggerbuggerbuggerbugger.......

So I pick up the mic and transmit (It can't possibly work, though).

After 3 or 4 seconds, smoke starts coming out of the dummy load (The VHF is pulling 5 amps while I transmit), so I finish off the message *very* quickly "...This is MMSI######## Cancel distress I will contact you by phone".

Get an acknowlegement loud & clear PHEW!

Now Holyhead is 70nm away as the crow flies, but even if they have an aerial in Llandudno (as I suspect), that's still 20 miles, and the thing in the photo above was in my back kitchen, on the wrong side of the house from that. I find it a very surprising result.

They were fine on the phone - took the opporunity to update the CG66 info, and wished them well with their pay claim /forums/images/graemlins/blush.gif

So, now I know I've got the right phone number for the coastguard in my phone; my CG66 is up to date, ....oh, and I know the DSC alert works /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif

Suitably chastened.

Andy
 
I'm more surprised that you received their reply. Consumer radios re often quite poorly shielded, enough signal probably came out of the side of the box. Or the dummy load grounding isn't good enough. A coastguard antenna will have a lot of gain, its radiation pattern is focussed horizontally. There is a vast difference between the range that is always going to work and the range that is never going to be intercepted. Otherwise GCHQ wouldn't work.
Dare I say you should have asked for a radio check first?
 
[ QUOTE ]
Dare I say you should have asked for a radio check first?

[/ QUOTE ]

/forums/images/graemlins/grin.gif

I probably should have, but would never have believed that anything would get in or out - especially perfectly readable speech.

I think I need to screen the dummy load and twist the supply cables together, etc. then check I can't get anything on the hand-held if I try again.

Andy
 
[ QUOTE ]
am surprised it would take 20 watts without frying.

[/ QUOTE ]

Good call - it's only good for something under 10W continuous, but I figured it would be OK for a quick burst of 25W from a DSC transmission. It didn't like the subsequent extended conversation, though. (The original plan was to stick it in an oil-filled can - not sure if I'll bother now!)

Andy
 
In a previous life Iused to get a bit involved with radio stuff, including getting consultancy from the guys who 'invented' leaky feader for use in tunnels.

From this it would appear that one can do the various sums and end up with the theoretical performance of any antenna in any situation and be reasonably sure that that is what you will get, but you can always get extra, and whilst some of the extra (the stuff you get from ducting for example) can be predicted some just happens with no apparent scientific reason.

Put simply if you let RF out into the atmosphere sh*t can happen.
 
[ QUOTE ]
A coastguard antenna will have a lot of gain, its radiation pattern is focussed horizontally.

[/ QUOTE ]

Is that correct or do you mean vertically?

I have always assumed (maybe wrongly) that the radiation pattern would be vertical as the majority of yachts and motor boats, (ships?) have their antennas mounted vertically.
 
Horizontally. VHF antennas achieve their 'gain' by focusing the radiation pattern. By focusing it at the horizon and not at the sky or the ground they achieve greater signal strength at a given distance.
Yachts use VHF antennas with relatively low gain, more like a fat doughnut radiating from the masthead than a flat disc, so that as they roll and pitch a greater portion of the signal is pointing at the horizon.
3dBi gain 1m whips are almost always used for masthead antennas; by contrast a 9dBi antenna would be around 20' tall and have a very flat, focused radiation pattern.
 
It's a dummy load made up of 2K2 resistors. If it's 50 ohm then there will be 44 resistors. They look like half watt rating so the total rating is 22 watt. No problem for short term use on a marine vhf.
You've got to admire the patience of the person who soldered all those resistors on a bit of veroboard.
 
[ QUOTE ]
...You've got to admire the patience of the person who soldered all those resistors on a bit of veroboard.

[/ QUOTE ]

Really, it was nothing...... /forums/images/graemlins/smile.gif (7 at a time, straight off the bandolier)

You've sussed out the design correctly - 44 x 2k2. I think they're 1/4 or 1/3 watt (old), and packed in a bit tightly, so they warm up very quickly, as you might expect, but they're what I had lying around.

Andy
 
It certainly surprised me !

I had it running off a bench power supply, with two separate (long) patch leads for power - that's probably where the signal came from (or the unscreened load). The radio has a ferrite bead on the power leads where they leave the set, but guess that's only there as an afterthought to reduce the stray emissions enough to get type approval. The radio casing is plastic, too.

Nothing was earthed /forums/images/graemlins/blush.gif

I've had the set on receive since, and it doesn't pick up the routine transmissions from Holyhead (can just about hear them with the hand-held), but they certainly came booming through after the DSC alarm - I guess they would have turned up the wick on a local aerial?

Andy
 
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