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Old 16 July 2007, 07:42   #1
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VTronix Rib Raider - help

Just installed my new Rib Raider yesterday and as I always do with these things I prodded the plug with a multimeter to make sure it was OK.

According to the meter the outer and inner cores of the coax are connected. It isn't a soldering fault in the plug (because I cut it off and checked it then re-did it and the plug/coax is fine without the antenna connected) and it isn't the obvious fault that you can create when over-tightening the connector on the antenna end and shorting it there.

I have seen VHF amateur aerials that "appear to be a dead short" and work fine, and I have never understood how this can work, but it does. So I wondered if somebody with a Rib Raider would be kind enough to pop their plug off the back of the radio, check the resistance from centre to outer on the coax plug with a meter and tell me if theirs in the same?

The radio seems to work on send and receive (tried it at about a mile into a radio in somebody's vehicle) but I am just wondering if there is an internal fault in the aerial or whether it is supposed to be like this...

Thanks
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Old 16 July 2007, 12:44   #2
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Stephen,

The only sure way to check if a radio antenna is working properly is to use a SWR meter which will measure its performance. You can't really check an antenna with a multimeter.

If your transmit works OK, then the antenna is working fine (Receive is possible with a coat hanger or similar, but will possibly stuff your radio when you try to shoot 25W up it!)

There's others on here who I'm sure will provide chapter and verse on SWR readings!

Mike
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Old 16 July 2007, 13:08   #3
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Hi Stephen,

I happen to have a Rib Raider antenna in front of me, and a multimeter right next to it, so it seemed lazy not to try it! And yes, the core and shield are connected.

Now it'll be up to somebody to explain baluns, SWR, dipoles and other such arcane matters while I glaze over ;-)

Jon.
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Old 16 July 2007, 17:40   #4
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I had exactly the same concerns as you when I fitted my Raider antenna but more in terms of did I short out the plug at the radio end when I soldered it. The screen on the cable itself is completely seperated from conductor so if you disconnect the antenna end it should be open circuit.

The Antenna will most likely appear as a dead short as it's impedance not resistance that matters - impedance being effectively the resitance when operating at the correct frequency. There are some great (!) equations to work it all out if I could remember my electromagnetic theory!

Best thing to do is see if you receive OK and that will tell you if it's all working OK with no dead shorts. Transmitting into a short circuit will most likely damage the radio as mentioned above. I seem to recall you were talking about getting a Std Horixon Handheld so just try transmitting with that and see if you receive OK on the main set. I did then then moved onto a quick radio check between main and handheld at 1W to satisfy myself everything was OK before transmitting on 25W.

Seems to be a good antenna. I've been using mine for 6 months and am really happy with the results so far.
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Old 17 July 2007, 07:45   #5
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Thanks guys. Haven't tried my SWR meter as I ran out of daylight, may do on the weekend if it is fit to work outside (mid winter here remember!)

It was the possibility of a dead short that concerned me but it seems to work fine (quick test) on both transmit and receive to a mobile unit about a mile away and not line of sight, so I guess it is OK then. I also did the tx/rx thing with the HX270 but I have found that the Icom on the boat would receive a transmission within 100m or so even with no aerial so that wasn't conclusive!

I guess it is fine, must be the impedance/resistance thing, I don't understand it and don't need to as long as the ant is fine I just accept it works

Must admit though in my other radio activities I have always found that an "open" antenna (where the core is connected to the mast, and the ground to the mount) always works better than one like this. I also have seven or eight VHF amateur sets and quite a collection of whip aerials!
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Old 17 July 2007, 10:23   #6
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Mid winter here as well er i think
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Old 17 July 2007, 10:41   #7
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Stephen

It could well be that there is a matching transformer within the base section. This means that sticking a meter across the terminals of the aerial will simply measure the resistance of the primary coil of the transformer (probably only a few Ohms). As you've found out (and as al40 suggested) this doesn't knacker the performance of the aerial, in fact its there to improve performance.

HTH
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Old 17 July 2007, 13:02   #8
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Now you mention it the "shorted" ones do all have fairly chunky bases on them. I guess it's fine, so that is a relief. I still reckon that the "normal" ones give better performance on 2m though, not sure about marine band. Will pop the SWR meter on next weekend and see what is what...

Thanks
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Old 17 July 2007, 13:42   #9
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I should imagine the normal ones do provide better RX/TX but on top of an A frame they get so much whip on a RIB that they don't last too long. Therefore the RIB Raider ones are a compromise to provide the same emmisions in a small package. They should last the effects of being whipped around. But plenty of us have still lost these. Just make sure yours is properley secured to its base, I mean the steel antenna in to the base. (Its got a small plastic nut you can tighten up) because you can't get the antenna bit on its own and its £40 plus to get a new one.

I think I'm slowly filling the Solent up with bits that have fallen off my boats or just got lost overboard

Nick R.
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Old 17 July 2007, 14:04   #10
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You may find the following useful.

http://ecatalog.westmarine.com/zoom.asp?page=26

A 6db, 8ft. Shakespear, mounted as high as pos. on a locking, folding stainless bracket is the way to go for a RIB

Regards, T
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