V aerial

Quarter wave at 158mhz =..

300/158 /4 x.95 =0.45m which is the length of your antenna radiators…so it must be for Marine band..
http://www.garfnet.org.uk/joomla/index.php?option=com_content&view=article&id=95:vhf-marine-band-channels&catid=3:radio-frequencies&Itemid=59

Dave,

Thanks for that, which makes sense. I calculate the length of the quarter wave to be approximately 43cm which if my maths is correct would give a frequency of 174.3 MHz. If that’s right, does that give any more indication of the function of the aerial?

Normal half wave dipole is 2 quarter waves fed at the center (Highest current/I and lowest voltage/V ) gives good impedance(Z) for matching to TX output .

Usually in the rough calculation you go for about 95% of the calculated length to allow for “end effect” ie: the thickness of the radiator.

Dave,

My bad! Thanks for that. I’d always understood each dipole was a half, not a quarter wavelength, but there you go. I’ve definitely learned something new!

If this aerial is marine band VHF for aerial use, is it perhaps off a Shackleton?

I make the resonant frequency , calculated from your measurement of 45cm per quarter wave section(2 quarterwaves make a half wavw dipole) to be 158mhz approx.

The calculation …

45cm(one quarter wave) divided by 95 then times 100 (This removes the end effect allowance which will have been calculated in when designing the antenna)=47.37cms.

47.37 x 4=189.48 cm(1.8948m)

300 divided by 1.8948 =158.32 Mhz approx….

At a guess I’d say it’s a marine VHF band antenna….for airborne use.

Thanks for the posts. Ignoring the curved bit joining the two dipoles together, each dipole measures 45cm long. From that I figured a wavelength of 90cm which I calculate to a frequency of about 350 MHz (well, actually 333 MHz).

And, no I’m afraid it’s not a V-beam aerial! 🙂

Just for a moment, Ian, when I read your thread header I thought you might have been talking about REAL V-beam aerials! You know, a real, tactical, steerable, HF aerial. Hundreds of feet of R4 copper aerial wire all strung from the top of a tactical, air-transportable, 80 foot mast! And the Grainger log-periodic ditto. On one Det it had taken us 2 days (at Stornoway in the winter!) to get this beast up. We finally got in comm with some Fishead boat in the Caribbean with a howling gale blowing and horizontal rain. Suddenly, there was a loud bang and everything went dark and quiet. Investigation showed that one of the local sheep (soaking wet) had decided to ‘scratch an itch’ on the bottom of a live aerial leg. Bang! Dead mutton (still steaming!). Even knocked the Meadows genny off-line!
That, Ian, Old Friend, is a real V-beam aerial. Not your Toytown stuff!
HTH
Resmoroh

What are it’s dimensions ?

At first glance I thought it was off the front of an MC-130E 🙂

That was much more heavy duty though

If you have any amateur radio friends with an antenna frequency analyser, a good start would be to stick a bit of 50 ohm coax and a PL259 plug the antenna and find out just what freq it is resonant on.