HF Radio Aerial Experts Question

Submitted: Friday, Feb 05, 2010 at 16:56
ThreadID: 75802 Views:3511 Replies:6 FollowUps:6
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HF Aerial 1.8m Stainless Steel, as I am mainly tuned into 8022 MHz how much (cm) can I cut off the top of the aerial so that it will still auto tune to 8022 MHz and give the aerial a lower profile on the vehicle………..WHY because I want to!…….Radio is a Codan NGT with 3 Aerials, 2 Stainless Steel, 1 Fibre Glass, so I can always change the aerial to the full size if needed.
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Reply By: olcoolone - Friday, Feb 05, 2010 at 18:01

Friday, Feb 05, 2010 at 18:01
You can make the antenna as short as you want, but the shorter it is the less it will perform and the harder it will be to tune into lower frequency.

The whip antenna that is supplied is a short antenna to start with and is miles away from being the optimal length.

For example a 1/4 wave length for 8 Mhz is 29 feet compared with you 5 feet at the moment, so really your factory whip is about 1/12 of a wave length.

This will affect you transmit maybe more then your receive, on some radios when receiving a preamp will increase the receive performance.

I'm not sure how the NGT works but I know on the previous 93 series the preamp would only work on open tune.


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Follow Up By: Member - John (Vic) - Friday, Feb 05, 2010 at 18:49

Friday, Feb 05, 2010 at 18:49
The rest of the aerial "Wire" length is wound inside the Auto tune, in your example for 8 MHz and your optimal length of 29 feet he has a 5 ft S/S whip then the Auto tune will adjust the aerial length internally for the other 24 odd feet.

If you cut the whip down you may not have enough length within the Auto tune to adjust to the optimal length.
I hope that makes sense??

Me I would leave the aerial lengths as supplied by the manufacturer or you could find that it won't tune to some frequencies.



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Follow Up By: Member - Mike DID - Friday, Feb 05, 2010 at 21:40

Friday, Feb 05, 2010 at 21:40
Making the aerial resonant using the loading coil still won't make it as effective as a straight 29 foot whip.
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Follow Up By: olcoolone - Saturday, Feb 06, 2010 at 18:43

Saturday, Feb 06, 2010 at 18:43
The auto tune antennas don't have 24 feet of antenna inside, an auto tune antenna works like any standards antenna tuner.

The function of an antenna tuner is to effect a match between the output of a transceiver and the input of an antenna system. Modern transceivers can only deliver full power into a 50 O load. Antenna tuners are variable-impedance
transformers that allow you to transform the antenna system impedance (which can be almost anything) to 50 O for the transceiver. Some antenna tuners exhibit a wide impedance- matching range.

So in other words you are making the transceiver think it is perfectly matched to the antenna artificially.

As with most antenna tuners they can only tune so much and if the antenna is short for the tuner they will not tune.

An antenna tuner will not make a badly matched antenna work as well as a matched antenna.

A 5 foot whip on a auto tune base will be no match for a full length antenna .....if it was the case why would we still need big antennas when a short one would do just as well.

The difference between an auto tune antenna and a correct length antenna (1/4 wave length and up) could be anything up to 3 to 4 "S" points, to do this with just power alone you may need to increase the transmit power 2 to 3 times.
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Follow Up By: Member - John (Vic) - Saturday, Feb 06, 2010 at 19:29

Saturday, Feb 06, 2010 at 19:29
OK good info, Thanks I stand corrected on my understanding of the operation of the Auto tune, I have seen the long coil of copper wire inside them and thought by an explanation from someone else a long time ago (obviously incorrect) that they in effect shorted out this coiled wire to set the required aerial length.

I concede my understanding of the "S" points etc is zip :-)

Cheers



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Follow Up By: olcoolone - Sunday, Feb 07, 2010 at 09:37

Sunday, Feb 07, 2010 at 09:37
S = Signal strength from a scale of 0 to 30+, most signals are with in the 2 to 7 on average.

A station with a reading of S6 will be stronger then a radio with a S3 reading if using the same modulation level.

This is only a rough scale as it depends on Radios and S meter calibrations.

Much the same as a fuel gauge, it only gives you an idea and can vary from vehicle to vehicle.

John have a look on Wiki for a better understanding
http://en.wikipedia.org/wiki/Loading_coil
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Reply By: Member - Fred B (NT) - Friday, Feb 05, 2010 at 18:33

Friday, Feb 05, 2010 at 18:33
the aerial length is designed to give the best possible preformance for the set. As mentioned above, it will affect the preformance by shortening it. How much it is affected will be determinded by any number of factors.
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Fred B
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Reply By: MEMBER - Darian, SA - Friday, Feb 05, 2010 at 19:36

Friday, Feb 05, 2010 at 19:36
Agreed - we discourage your intention Cobber :-o)..... please don't cut anything!
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Follow Up By: MEMBER - Darian, SA - Friday, Feb 05, 2010 at 19:38

Friday, Feb 05, 2010 at 19:38
er..... should have added....I'm no HF expert.... just a seasoned user of such travel 'toys' :-o).
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Reply By: On Patrol & TONI - Friday, Feb 05, 2010 at 19:49

Friday, Feb 05, 2010 at 19:49
G'day cobber
your right to do what you want is respected by most here, but as said a performance reduction will be the result & why mess with that? unless specific needs dictate that to be?
HF, as you know, is long range communication, it seems counter productive to reduce that range!!!!!

Cheers Colin.
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Reply By: Member - Mike DID - Friday, Feb 05, 2010 at 21:43

Friday, Feb 05, 2010 at 21:43
You can't predict or calculate how short you can go at 8 MHz without knowing more detail about the internal loading coil.

It's quite possible to operate without any whip at all - it just becomes a continuously loaded antenna.

Just try it with a 1 metre whip and see how low you can tune.

You can always use the shortened antenna for receiving and for transmitting when signals are strong - then stopping and putting on the long whip only when you have to.
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Reply By: Member - Boobook - Saturday, Feb 06, 2010 at 06:47

Saturday, Feb 06, 2010 at 06:47
Have you thought about selling the Autotune and buying a tapped antenna. It will probably be lower than the autotune and you will end up with $400 in your hand.
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