VHF Aerial Experts...

[st599]

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

[/ QUOTE ]

Why? It looks like it's got about 50 1/2W resistors in parallel.

 

[MM5AHO]

That's actually not even a good dummy load - wrong type of resistors. The pictured type have some impedance, not pure resistance, so it might radiate more that you'd expect, especially with the cover off. On a good day perhaps even the 70 miles, dependent on the height it is at when TX.

 

[Spyro]

The coastguard will have lots of transmitters are you sure they don't have one near you?

 

[haydude]

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

Impressive, however a dummy load normally is soldered directly to the plug, without any wires. Now perhaps I understand why, it looks like that short wire worked as an effective antenna for your rig.

 

[st599]

[ 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.

You're getting radiation pattern and polarity mixed up.