Mounting 2.4m vhf antenna on a frame ?

[Orwell boy]

New to me rib has vitronix rib raider style anntenna on the top of A frame ,function seems poor if not zero so am considering replacement ( have seen regular failure reports on these and had same on my R/C 5.3 ) Best repair tips anybody ?
So previously replaced on r/c with 1.4 ish metre Shakespeare glass with st st ferrule ,and was super , so bought a nice shakespeare folding mount

Was thinking i would go 2.4m on side of a frame ,but no suitable free space ,only on top .........so question is 2.4m too much on top of a frame or are the transmit /receive benefits worthwhile ? thanks

[ShinyShoe]

Originally Posted by Orwell boy

Was thinking i would go 2.4m on side of a frame ,but no suitable free space ,only on top .........so question is 2.4m too much on top of a frame or are the transmit /receive benefits worthwhile ? thanks

Those seem like two different questions.

1. Is 2.4m on the A frame too much?

2. Are the transmit benefits worthwhile?

The two will be a balance of the two presumably.

If we assume the A Frame is 2m ASL you are doubling your height. That can only be good for transmitting/receiving.

Range:


1.23 * (√h1 + √h2)

(h 1&2 are the height of the two masts in feet, and gives a range in miles.

So lets assume you are now 13ft ASL and transmitting to someone stood at shore side with a radio on their waist 3ft, you range becomes 6.6miles, compared to 5.1mi.

Does that matter? Presumably depends if you need to talk to a guy on the shore 6 miles away.

Talking to coasties.. 16mi Vs 15mi.

But, at the same time there is a compromise of silly height when on a trailer etc. Your A frame must be 8-10ft from road which will go under most things. Adding 7 ft to that you are in low bridge territory. Same applies afloat. You'll know your area better than most to know how that works.

Personally I'd find having to fold stuff constantly a PITA.

[tehguy]

Why do you need that big an aerial on a RIB, how far off shore are you going? The coastguard here has their aerials 130m above sea level and you can easily talk to them with a handheld radio at 20 nautical miles (the limit for cat 3 operation). On our RIB we use Shakespeare 5241-R stainless steel whips, one each for VHF and AIS, these are specifically designed for fast boats as they have a much slimmer profile than a fibreglass aerial so far less wind load when you're doing 40 knots. I think they are 90cm tall and they sit on top of the A frame about 2m above the waterline.

[Orwell boy]

Thanks guys answered my question spot on ,will go for similar antenna as tehguy has i think ,thanks SS for the calcs sorted the theory side so sorted my reasoning
thankyou

[willk]

Originally Posted by Orwell boy

.........so question is 2.4m too much on top of a frame or are the transmit /receive benefits worthwhile ? thanks

You might check the dB on those - some of the long units are 6dB and the "transmission plane" (my term) can be a little flat for those in a bouncy craft with odd angles of incline.