I know nothing about ariels/antenna! Help!

M

matnoo

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Joined
6 Jun 2006
Messages
171
Location
Solihull, W.mids
www.faceparty.com
Whats gain, whats resistance, whats a ground plane for and why might i need it!? Whats SWR? Why do some have a big 'lump' (called a coil?) at the bottom and some not?


Im not after anything fancy I dont think, im not going to go far out to sea at all for a good long while as im still a novice, I have a cobra £120 (non portable) type radio that transmits up to 25W.

thinking of this one?

https://secure.mailspeedmarine.com/Produ...fc-bb74d9d5a564

cheap and comes with cable... any good?

Mat
 
Gain is the increase in power ratio against a known performance antenna (e.g. half-wave dipole), usually expressed logarithmically.

Resistance is d.c. Impedance

A ground plane allows an unbalanced antenna to operate correctly, rather than looking like an open stub.

SWR is a measure of the ratio between peaks and nulls in a standing wave pattern.

The coil is used to electrically lengthen an antenna.
 
[ QUOTE ]
Gain is the increase in power ratio against a known performance antenna (e.g. half-wave dipole), usually expressed logarithmically.

Resistance is d.c. Impedance

A ground plane allows an unbalanced antenna to operate correctly, rather than looking like an open stub.

SWR is a measure of the ratio between peaks and nulls in a standing wave pattern.

The coil is used to electrically lengthen an antenna.

[/ QUOTE ]

Well, that's cleared that up then /forums/images/graemlins/grin.gif
 
[ QUOTE ]
Well, that's cleared that up then

[/ QUOTE ] yep now as clear as mud in a wine glass
 
Gain help(?)

Hi,

Think of the output of a radio like the old RKO logo (am I dating myself?), circles coming out of a single point. That cant happen in real life, so...

Imagine blowing up a perfectly circular ballon. The outside of the circle is the power that the radio transmits. If, for instance, you take your two forefingers and press them into the ballon so that the tips meet. The ballon is now *skinnyer* (is that a word?) where your fingers are and is fatter 90 perpendicular to your fingers.

The difference between the original shape of the ballon and where it bulges to after you have pressed with your fingers in the so-called gain: You are not gaining any energy, you are redirecting it into a different direction.

In the above example with the fingers, the shape you see is what you would get from a dipole-type antenna. On a VHF radio such as you are considering, you would mount the antenna so that the fat part is paralell to the water surface so that the signal doesnt radiate into the sky and the null (where your fingers come together) isnt important unless you are listening to airplanes or submarines.

Is this better?
Best regards,
Michael
 
It should be noted that Gain in it's true sense ie "power amplification" doesn't happen in an antenna system, as it is a passive component.
An antenna can only direct power in a certain direction, using the power that would have normally been directed either behind it or upwards/downward.
A 5/8 wavelength marine antenna will typically give improvement between 4 and 6dBd by using the power that would have been radiated at angles above 45 degrees and radiating it sideways.
A very good Beam (yagi) antenna can achieve typically up to about 20dBd by having a very narrow beamwidth in a single direction with virtually no radiation behind or sideways.

Steve
 
Marine VHF aerials are designed to have some metal below them like a mast or similar to act as the ground plane. The normal wiring of many marine aerials makes them look like a short circuit to an ohm meter. This makes them into lightning conductors, discharging the static before you get hit, the static goes to earth via the metalwork and the DC short circuit. Thats instead of ending up inside the VHF.

The VSWR meter is only really needed if the antenna is suspect. If its new, fitted to some metalwork, and you havent damaged the wire and you have the plug properly fitted to the end of the wire theres not a lot to worry about. Once its 20 years old it may be corroded and need replacing.

It took about three weeks to realise that there was a blob of solder shorting the cable in a newly-installed yacht VHF aerial because we only ever used it in a racing context over a range of less than a mile. Then I used the VSWR bridge and an ohm meter and found the problem. It was the connector at the mast base, recently replaced along with the mast.

The moral is to do radio checks over a distance of more than a mile. Calling up the marina while you are in it is not a good check.
 
Dont worry about any of this stuff matnoo - even if you understood it, you wouldnt be able to do anything different in practise. for a marine vhf, do what the rest of us do, go out and buy a standard marine vhf aerial. for practical purposes, they are all the same performance wise.

only time you need to think of the swr is when your aerial goes duff and more of your transmitters power is reflected back into the radio than escapes from the aerial. then your swr has gone high and your aerial is b***ered (another techie term). but people will simply say "i cant hear you" rather than "your SWR has gone high, old fruit"
 
Now, can any of the esteemed experts here help me work out how to have a DAB antenna, and a VHF (FM radio) antenna mounted on the soon-to-be-fitted strut above my radar? The antennae available commercially have cabling which is much too short...

I can adapt a commercial (car windscreen) DAB antenna by mounting it in a plastic tube, but how to wire it? And can I simply use 75 ohm co-ax to connect an FM antenna with success?

All assistance much appreciated...
 
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