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[Martin (Admin)]

BTW... Can anyone tell me what the receiver aerial length should be for 27Mhz please?

[Colin Bishop]

Around 36 inches is about average. It's not all that critical.

[Martin (Admin)]

So what side of 'critical' is 13"?  ::)

Thanks Colin.... Alright, where's me soldering iron?

[Peterm]

Martin, a lot depends on the range you want for your boat.   One of mine has a 12 inch ae and works fine up to 200 metres.   Pete M

[andrewh]

Martin,

Me, I'm a simple mech Eng.  one of the evident things in life is that if one squngles the aerial up in a boat near the water the actual length of wire becomes  fairly insignificant. 
All my rxs have a socket fitted asbout 2 inches from the rx and areial wires fitted to the vehicle as near the original 30 to 36 inches as reasonably possible. 
I use transformer wire for the aerials - infinitely thin in the case of 10gm planes!
For boats I try and get as much of the wire vertical as possible, or at least as high as possible
andrew

[dreadnought72]

Martin, I've always followed this approach:

1/ Speed of light (c) is 3*10⁸ m/s
2/ Frequency (f) is 27*10⁶ Hz
3/ Since wavelength = c/f
4/ Wavelength = 11.1 metres
5/ Aerials pick up harmonics (1/2, 1/4 etc)
6/ 1/8th of 11.1 metres = 55"
7/ 1/16th of 11.1 metres = 27"
8/ Most 27MHz receivers have aerial lengths nowhere near these lengths.

Therefore:

9/ Don't cut it if you can avoid it. While the magic smoke doesn't come out if you do cut off the aerial, the etheric imps can't find out how to get into the receiver.
10/ Try to stick it vertically.

Andy, bemused and adrift in radio.

[Colin Bishop]

Try to stick it vertically

Up where? 

[andrewh]

Thank goodness, at last the exact science behind it all

Sadly disappointed that ANDY(999) has discarded most of the numbers after the spot :( but agree in spades with his conclusion

What are these aetheric imps?  I thought the stick positions were turned into rarifactions of a subtle imponderable radio fluid which is poured out of the transmitter (isn't that why people hold the transmitter higher in moments of stress?)

andrew

[malcolmfrary]

A lot depends on what the RX designer put into the front end of the RX circuit.  The gathering ability of the wire combined with the RX design is a balancing act between pulling in wanted signal and the ability to reject unwanted signal.  Extra length above that provided by the manufacturer normally does no harm, but equally probably won't do any good.
Whatever length, upright is best.  I saw a lifeboat at OWLS show that had a tubular aerial support that was painted white, but had some rather nice clips fixing it to the rail.  As a result, it stopped being an intrusion and became a plausible but little know prototype fitting.
It is possible to have an aerial wire that is too thin.  Desperately trying to remember some of the Telecommunications Principles I did 40 odd years ago, higher frequencies require larger diameter conductors due to something called the "skin effect" - the electric only uses the surface - so a thin wire might just as well not be there when presented with a high frequency.  27MHz is probably borderline OK, any higher frequency, use the same stranded as the original.

[malcolmfrary]

Try to stick it vertically

Up where? 

Up there.  Way up there. 
You need to give the imps the best start in their travels.

[Colin Bishop]

Malcolm 

It is possible to have an aerial wire that is too thin.  Desperately trying to remember some of the Telecommunications Principles I did 40 odd years ago, higher frequencies require larger diameter conductors due to something called the "skin effect" - the electric only uses the surface - so a thin wire might just as well not be there when presented with a high frequency.  27MHz is probably borderline OK, any higher frequency, use the same stranded as the original.
 

I never knew that! I've often used very thin wire strung up and between the masts to give the aerial an airing, so to speak, and not experienced any problems but that was on 27mhz. Latest boat is on 40mhz so maybe it would be better to use standard wire and run it round inside the superstructure instead.

You learn something new everyday on here. 

Colin

[dreadnought72]

Try to stick it vertically

Up where? 

Up there.  Way up there. 
You need to give the imps the best start in their travels.

Correct, and you do.

Andrewh is right to involve the imponderable radio fluid - the imps drink it. Previously, it made them a bit wobbly, and having to remember the numbers of stick positions in their heads, they could frequently get it wrong at long ranges or if they chatted to other imps nearby. These days, the imps are made to use their fingers to record these numbers, and mistakes are fewer.

Hence digital radio.

Andy #1963

[andrewh]

 Desperately trying to remember some of the Telecommunications Principles I did 40 odd years ago, higher frequencies require larger diameter conductors due to something called the "skin effect" - the electric only uses the surface - so a thin wire might just as well not be there when presented with a high frequency.  27MHz is probably borderline OK, any higher frequency, use the same stranded as the original.

Very true - the higher the frequency the closer electrikery stays to the skin of the conductor - but from this basic fact I have drawn (what seems like) the opposite conclusion:
Of the current flows only in the skin, it is not flowing in the core - so there is no point in having a big or heavy core - hence use the thinnest possible wire!

Or am I missing something?
andrew

[andrewh]

Andy1963
Is it true that there are now 10 kinds of the digital imps?
andrew

[Colin Bishop]

I think Malcom's point was about maximising surface area to pick up the signal but I know nothing about this subject.

Colin

[sweeper]

Re: skin effect.
You do not require high frequency for this effect to occur. It is a known effect on 50Hz systems.
Many people assume that the current flow through a conductor is uniform across the whole cross section of the conductor  while it is actually biased towards the outer surface of the conductor. There is a rating factor applied to power cable calculations to allow for this.
That should mess with a few brain cells! ???

The simple explanation is that the larger diameter of the conductor (close to the surface) produces a lower resistance to current flow than the smaller diameter nearer to the centre of the conductor.
It is one (note: one) of the reasons that some high voltage systems employ hollow conductors in the form of tubes.

Hope this helps someone.
Regards.

[Martin (Admin)]

OK, what effect does wrapping the aerial wire into a coil have, (to reduce length)?

[DickyD]

No , it makes it curly. 

[wideawake]

OK, what effect does wrapping the aerial wire into a coil have, (to reduce length)?

Making some of the aerial into a coil effectively makes that part into an inductor with an aerial attached on top.   It's the way both transmitters and receivers get away with shorter than optimum areials (optimum for an end fed aerial is generally 1/4 wave).   To see this in a very visible form google for Springer CB aerial where the coil is exposed.  More usually it would be wound of finer wire and hidden.   It's not OK to just wind part of an existing aerial into a coil as it won't be properly matched any longer.   The aerial part would need to be trimmed using th e right equipment to see when it matched again.   To be honest you'll probably not do any damage to a receiver if you shorten the aerial a bit , just reduce its sensitivity whereas on a transmitter you could damage the output stage by operating with the rod aeral collapsed unless it's protected.   Isn't it generally easier to just leave the spare loosely under the deck?

Cheers

Guy

[Bill D203]

Hi Martin
Use 2.4 Ghz radio and you only have two very short Ariel's to worry about.

[malcolmfrary]

Of the current flows only in the skin, it is not flowing in the core - so there is no point in having a big or heavy core - hence use the thinnest possible wire!

Or am I missing something?

The electric wants the largest cross sectional area, but is reluctant to use the middle bit, so a thin wire is effectively all middle with no outside skin.  Like I said, I did my learning a long time ago, and a lot of dust has settled on it.  Certainly at radar frequencies the conductors are tubes, there being no point in having anything in the middle.  If you go shopping for coaxial cable, you tend to find that the higher the frequency, the fatter the cable to minimise the losses.  Using a single fine conductor will be OK up to a certain (in my case, UNcertain) length at any given frequency.  Beyond that length, anything that the extra picks up will be lost by the time it gets to the RX.
Coiling.  It depends on the coil.  It is possible to wind up with a loading coil - in some circumstances this can be beneficial, but most likely not.  A loose, large diameter coil allows the imps to hurtle round in a wall of death like manner, and arrive at the RX.  A tight, tidy coil will cause them to fall off.  Not actually true, but enough for general understanding.  My Lindberg Tuna boat has the antenna coiled, about 1.5" diameter.  This is fixed vertically in the bow, mostly above the waterline.  Over 50 yards range.  More than this, I can't see which way its pointing, so any loss of range is immaterial.  The same radio with the aerial vertical in a yacht,400 yards.

[FullLeatherJacket]

I was very tempted to write "suck it and see" but having read some of the other comments I thought that would be an undesirable phrase to use

I also put a small plug and socket in the flexible aerial lead; cut it down in length and connect the end to a vertical whip aerial, and so I go with the theory that the manufacturers must know at least as much as I can find out. The combined length of the bit left on the Rx plus the bit between the plug/socket and the bottom of the whip aerial plus the aerial length is as near as dammit the same as the original. 7/0.2 flex does fine; 10/0.1 will do. 18 or 20SWG piano wire for the whip aerial, and off you go. Don't forget the anti-poke-in-the-eye bead or loop on the top. Shout if you want a sketch, huh?

Works for me on 27 and 40 Meg.

FLJ

PS This does NOT apply to 2.4 Gigglebyte sets! RTFM and leave their aerials well alone.

[dreadnought72]

The electric wants the largest cross sectional area, but is reluctant to use the middle bit, so a thin wire is effectively all middle with no outside skin.

...a collection of thin wires with the same cross-sectional area as one fat wire would have more skin and less middle. So even more, extremely thin, wires would be better? But the tx aerial is just a tube.

I'm confused. I'm going to stick with the imps.

Andy #1963

[malcolmfrary]

The electric wants the largest cross sectional area, but is reluctant to use the middle bit, so a thin wire is effectively all middle with no outside skin.

...a collection of thin wires with the same cross-sectional area as one fat wire would have more skin and less middle. So even more, extremely thin, wires would be better? But the tx aerial is just a tube.

I'm confused. I'm going to stick with the imps.

Andy #1963

This is where the black art bit creeps in. 
With enough ouzo, any theory can be made to fit any collection of facts, at least until the ouzo wears off. 
The bundle of thin wires are uninsulated from each other, so they are effectively one thicker wire.  Yet more wires better?  Maybe, but not to the point where you would notice.  Like FLJ says, the manufacturers usually know something. 
The TX aerial is a tube because of mechanical considerations rather than anything electronic.  It is best as a straight line, this is most easily arranged using a rod, which needs the diameter to be self supporting and being hollow keeps the weight down.  For convenience, they usually telescope away, which again implies a tube.  You could take the same length of stranded wire and dangle it from a helium balloon to hold it vertical, and it would work as well. 

[GARY C]

Hi Martin,  you shure have opened a can of worms on this one, as the question can take you many ways  ???  :-\

In my younger years I used to do a bit of Ham, Amateur radio. To make a quick aerial for 27 mhz you could simply get a long lenth of any coax cable, and at the oposite end to the transmitter/reciever you would split and seperate the outer from the inner to a certain length. then lay it out into a T shape, this would then be hung horizontally up on any building. ( my name is not James Bond and I do not work for MI5 /6 )
I do not have any reference books now, or remember the formular of measurement. BUT this measurement did not have to be critical. you should find some answers if you google the question.

E.G. 27mhz band falls between 10m and 12m amature bands, total lengh adds to 232mm.

All my boats DO NOT have the reciever wire cut, on either 27 or 40 mhz. I just connect it to a piano wire style aerial or pass up through a tube style aerial. In all my years of model boating this has never been a problem with racing in OMRA, scale or sail. Please note as we all should know this DOES NOT apply to any 2.4G radio (another can of worms)

Hope this is of some help, Gary.