UHF antennas (antenn'ae???).. again;-)

Submitted: Monday, Apr 14, 2008 at 23:30
ThreadID: 56650 Views:1844 Replies:6 FollowUps:6
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G'day all,
Just having a squiz at the Mobile One link that was posted up here a couple of weeks ago, and my curiosity has got the better of me;-)

In the section on raised feed antennas, it says "DON'T use antennas longer than supplied - you WILL damage your transceiver" ...
and "The only antennas that work with raised feeds areS474 style, M474 stye, and FDW's. any antenna top longer than 24" or 620mm WILL damage your radio"

Would any of you radio-savvy folk out there care to comment on this assertion? .. fact or fiction? .. if so, why is it so?...

Thanks in Advance;-))

Regards, Ed C

Confucius say.....
"He who lie underneath automobile with tool in hand,
....Not necessarily mechanic!!"

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Reply By: Nomadic Navara - Tuesday, Apr 15, 2008 at 00:02

Tuesday, Apr 15, 2008 at 00:02
The whips supplied with these antennas are as long (as high gain) as I would use. These high gain antennas rely upon being completely vertical so that all the segments between the phasing coils are in line and transmit together in phase with each other. If they do not work together properly you will actually get less gain than the smaller ones. I am yet to see one of these longer ones that does not bend back with the wind as you increase speed. When they lean back you loose much of the power up and down the road that you would have had when you are stationary.

PeterD
ps you will find antennas on radios and antennae on insects.
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Follow Up By: obee - Tuesday, Apr 15, 2008 at 09:19

Tuesday, Apr 15, 2008 at 09:19
cos insects have two of them i.e. plural

owen
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Follow Up By: Nomadic Navara - Tuesday, Apr 15, 2008 at 15:41

Tuesday, Apr 15, 2008 at 15:41
Antennae is not the only plural of antenna. You have got to know what you are talking about before you can understand. Consult a dictionary and not one produced by the septic tanks.

PeterD
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Follow Up By: obee - Tuesday, Apr 15, 2008 at 18:36

Tuesday, Apr 15, 2008 at 18:36
Maquarie:

antenna /an'tena/, n., pl. -tennae /-'tcni/ for def. 1; -tennas for def. 2. 1. Zool. one of the jointed appendages occurring in pairs on the heads of insects, crustaceans, etc., often called feelers. 2. a radio or television aerial. [L: a sailyard] - antennal, antennary, adj. -antenniform, adj.

Chambers:

antenna, an-ten'a, n. a feeler or horn in insects, crustaceans, and myriapods: in wireless communication, a structure for sending out or receiving electric waves: an aerial:- antenn'ae (-e), antenn'as (radio).-adjs. antenn'al, antenn'ary; antenn'iform; antenniferous.-n. antenn'ule, one of first or smaller pair of antennae in crustaceans.

Yep. Antennas for British and Australian

The diversity of English is inspiring but I need to get out a bit.
Owen
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Reply By: Member -Signman - Tuesday, Apr 15, 2008 at 09:29

Tuesday, Apr 15, 2008 at 09:29
G'day Ed
I'm a bit of a tech-head when it comes to that- but I can't answer your question.
I do a bit of work for Mobile One, and I do recall Peter (the designer there) mentioned that to me. I didn't get a reason, but if you have no success here, Ill ask him next time I see him.

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Reply By: obee - Tuesday, Apr 15, 2008 at 09:29

Tuesday, Apr 15, 2008 at 09:29
put simply, antenna length is a function of signal wavelength. HF has a long wavelength and uses antennae cut to suit. You can have like full, half, 5/8th, 1/4 plus a bit of allowance for the entire physics of the things.

vhf and uhf are getting higher up the scale of frequency and have shorter wave lengths. Next time you go to the micro wave look at the little holes in the screen door. they are just a bit smaller than the wave length of the magnatron ouput so the radio waves cant escape but you can look in. Light waves are the uppermost end of the spectrum

Roughly speaking the transmission wave reaches max power as it fills the antenna. If not then it will feed part back to where it came from and turn into heat in the final trannies. Cooked trannie = dead radio.

I was never much good at electronics but I scraped thru the amatuer radio licence. It's fascinating stuff but I forget a lot of it.


Owen
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Follow Up By: Nomadic Navara - Tuesday, Apr 15, 2008 at 16:00

Tuesday, Apr 15, 2008 at 16:00
obee

End effect shortens the required length, not lengthens it. The thicker the conductor, the shorter it is for an electrical equivalent length.

PeterD
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Follow Up By: obee - Tuesday, Apr 15, 2008 at 18:43

Tuesday, Apr 15, 2008 at 18:43
Yeah I should have said plus or minus or even including. but it was the physics of thing I was referring to not end effect of which I have forgotten with a lot of other things. I was just trying to make it sound simple. I guess I got the rest of it right then.

Owen
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Reply By: Member - joc45 (WA) - Tuesday, Apr 15, 2008 at 12:57

Tuesday, Apr 15, 2008 at 12:57
Hi Ed,
the raised-feed antenna is effectively a dipole, with the coax feeding RF up the inside of the tube section, then feeding both up the whip and down from the top of the tube. A dipole has a different feed impedance (impedance being a measure of loading) at this junction to that of a ground plane antenna (the normal whip fed at the base of the antenna). Most multiple-wavelength whips as shown in the article are designed for feeding at the base of the antenna. Putting one of these onto a raised-feed antenna will create a mismatch, being the wrong impedance and this will cause reflected power back into the transmitter (a high VSWR). The result is both poor radiated power from the antenna and heat generated back in the transmitter.
Most transmitters are protected against high reflected power, but the protection is more designed to cope with mistakes or broken antennas rather than to cope with continuous operation.
Gerry
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Follow Up By: Nomadic Navara - Tuesday, Apr 15, 2008 at 16:37

Tuesday, Apr 15, 2008 at 16:37
All of the pre tuned whips are manufactured to mount on the same base and thus mount on the same base impedance. These are generally designed to operate with the base mounted on a sheet of metal with a diameter of one half wavelength or greater.

If you do not have a suitable ground plane to mount the base, you can create a whip mounting by using a 1/4 wave length (electrically) piece of tube below the feed point. Back in the steam radio days when frequencies were generally lower, we used these 1/4 wave pipes with a 1/4 wave whip on top - we called these co-axial dipoles. when hi-band VHF came into use and antennas were smaller we coupled a dipole to to the top of the whip (with coupling/phasing coils) and constructed an antenna with more gain. With the arrival of the UHF bands we could couple even more dipoles on the top but the basic design of the co-axial mounting sleeve did not change - it just got smaller with the increase in frequency.

It does not matter how many phased collinear dipoles you construct or whether you mount them on a conventional base or a co-axial sleeve, the impedance at the mounting point is the same. The biggest problem is keeping the elements in a straight line - this is impractical at speed with a vary long whip. I consider this is the important consideration for mobile use.

PeterD
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Reply By: Member - Mike DID - Tuesday, Apr 15, 2008 at 13:51

Tuesday, Apr 15, 2008 at 13:51
There is no excuse in a modern transceiver to have it fail due to antenna mismatch.

Even if you install a perfectly matched antenna, all sorts of things can go wrong that will result in a high SWR situation.

I would steer totally clear of a 5 watt transceiver that was not immune to infinite SWR.

I have never had a transceiver fail from high SWR - or the antenna falling off - and that's 100 watt transceivers.
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Reply By: Member - Ed. C. (QLD) - Wednesday, Apr 16, 2008 at 00:03

Wednesday, Apr 16, 2008 at 00:03
Many Thanks fellas, for your replies.. Much appreciated:)

Prob'ly won't be in here for a few weeks, so I'll
Catch ya's later...................

Regards, Ed C
Confucius say.....
"He who lie underneath automobile with tool in hand,
....Not necessarily mechanic!!"

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