Auto tune antenna length, bigger better?

Submitted: Tuesday, Jun 01, 2010 at 17:11
ThreadID: 78971 Views:4951 Replies:4 FollowUps:8
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I have a 9350 autotune and at the moment I am running a s/s whip which is about 1m in length. I see a lot of tourists getting around with the s/s or fibreglass whips which are about 2m in length.
My question is, do these longer antennas make a difference? The way I understand the autotune to work is that it adjusts the internals so the antenna which is supposed to be say 20m is all internal except for the 1m on top. So does having another meter of the "exposed" section make a reasonable differance? The only reference I have found to shorties before is for emergency use (not for everyday use??).
While I haven't tried it, I am thinking that maybe the shorter antenna reduces my frequency coverage at one end of the spectrum either 2Mhz of 30Mhz slightly reducing my capacity to transmit on these frequencies.
Any input appreciated.
Tim
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Reply By: MEMBER - Darian, SA - Tuesday, Jun 01, 2010 at 17:56

Tuesday, Jun 01, 2010 at 17:56
More wire is always better on my Barrett 950 system (910 autotune) - moving from the shorter of the two stock SS whips to the longer (2m) whip is an improvement, while use of 8.8m of wire draped over a nearby tree is great - I'm no tech, but more suitably orientated exposed wire in the mix seems the rule.
AnswerID: 419148

Follow Up By: blue one - Tuesday, Jun 01, 2010 at 18:50

Tuesday, Jun 01, 2010 at 18:50
Yep a 2 metre s/s whip works for me.

Cheers
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Follow Up By: Ozhumvee - Wednesday, Jun 02, 2010 at 08:56

Wednesday, Jun 02, 2010 at 08:56
Yes a horizontal antenna will always work better than a vertical one.
When camped up a 2 meter length of wire tied to a handy tree with nylon rope works really well off the top of an autotune.
Peter
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Follow Up By: blue one - Wednesday, Jun 02, 2010 at 10:24

Wednesday, Jun 02, 2010 at 10:24
Top of the the whip or auto tune body?
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Follow Up By: MEMBER - Darian, SA - Wednesday, Jun 02, 2010 at 10:43

Wednesday, Jun 02, 2010 at 10:43
Screw the whip off and hook the wire onto the autotune's screw. I carry an 8.8m length of thin insulated wire (probably 8A or such) - it suits 8022MHz, but will still work down and up a bit on the channel scale - for 14M+ frequencies I just go back to the long SS whip (the autotune baulks at high frequencies on long wires). I have a crimp on ring soldered to each end of the wire - one end simply hooks over the autotune's screw - the other takes an ocky and goes into a nearby tree........ re orientation..... the ideal seems to be having the wire elevated (30-45 degs etc.) and pointing away (180 degs) from the station you are trying to reach ! I'm told most of the signal comes off the side of the wire, hence a strong outgoing to the ionosphere, in the direction you want to go. I'm also told that if there is nothing to hang the wire on, trailing it along the ground, still pointing away, can be effective. If time permits, tossing a rope over a tree branch and pulling the wire up can be worth the effort (you have to have something to do, while the fire gets going :-o). Re safety and 'wireplay' - make sure nobody is touching the wire when you hit the talk button ! Lot of energy going out !
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Follow Up By: Mike DiD - Wednesday, Jun 02, 2010 at 23:02

Wednesday, Jun 02, 2010 at 23:02
"Yes a horizontal antenna will always work better than a vertical one. "

No, it depends on what distance you're working. If you're communicating under 500km you want a horizontal antenna.

If you're talking East Coast to West Coast you want a vertical antenna.
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Reply By: Robin Miller - Tuesday, Jun 01, 2010 at 18:52

Tuesday, Jun 01, 2010 at 18:52
Hi Tim

It is correct that up to a certain length of whip (about 2.5m ) for bands under discussion the increasing the length of the whip improves the transmission.

Its a bit frequency dependant - but a rough summary is that going from 1m to 2m will double the output of your transmitter, this is because your antenna is only maybe 5% efficent , i.e. 95% goes up in heat as the matching coils tried to put the transmitters power into the very low radiation resistance of your short whip .

As your aerial gets longer and approaches 1/4 wavelength (roughly 2.5m at 30mhz) it needs less matching and the efficency rises to 70% or more.

Hence a multitap often outperforms an autotune.

The quality (effective Q) of the autotunes components and the rest of the installation (grounding) deliver secondary effects , but put simply the longer the better.




Robin Miller

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AnswerID: 419152

Reply By: rescue134 - Tuesday, Jun 01, 2010 at 19:09

Tuesday, Jun 01, 2010 at 19:09
Tim
on my 9350 i have found the long s/steel whip tunes in better in the lower fregs 2020
around town (not traveling)i will use a 1mtr whip, but once on the roadi put on the long whip.
Kevin
AnswerID: 419155

Reply By: olcoolone - Wednesday, Jun 02, 2010 at 16:32

Wednesday, Jun 02, 2010 at 16:32
Tim one thing everyone forgot is a auto tune antenna does not have any wire inside to substitute the antenna length, what you have on the outside is what it is what you use.

On auto tune antennas the longer the antenna is the better.

On 8022 Mhz a 1/4 wave antenna is about 9m in length, so a full wave is about 36m...now take it down to 2 Mhz and it changes to about 31m for a 1/4 wave up to 120m for a full wave.

The antennas that come with auto tunes may only be 1/20 of a wave length for the longest whip on 8022 Mhz.

All a auto tune antenna does is fools the transceiver into thinking it has a matched antenna to transmit through, an auto tune does not act as a substitute for a right length antenna or increase performance.
AnswerID: 419252

Follow Up By: Mike DiD - Wednesday, Jun 02, 2010 at 23:06

Wednesday, Jun 02, 2010 at 23:06
A Codan Autune has a long coil of wire inside and its effective length is adjusted by moving a ferrite or aluminium slug inside it.

A Barrett Autotune switches in additional wire wound onto donut-shaped ferrite toroids.
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Follow Up By: olcoolone - Thursday, Jun 03, 2010 at 09:58

Thursday, Jun 03, 2010 at 09:58
So... the wire inside is a radiator?

Didn't think a choke was classed as a radiator.

So I take it the Codan autotune will increase the length of the antenna mechanically.

So using you theory I could transmit comfortable with out a antenna connected to a autotune and it would make very little difference.

And i take it this was wrong as well....
"All a auto tune antenna does is fools the transceiver into thinking it has a matched antenna to transmit through, an auto tune does not act as a substitute for a right length antenna or increase performance."

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Follow Up By: Mike DiD - Thursday, Jun 03, 2010 at 12:52

Thursday, Jun 03, 2010 at 12:52
"So... the wire inside is a radiator? Didn't think a choke was classed as a radiator. "

How do you think a tapped-whip antenna works at its lowest frequency ? It's a long coil - just like a choke.


"So using you theory I could transmit comfortable with out a antenna connected to a autotune and it would make very little difference. "

Yes, a Codan Autotune will tune up at the higher frequencies. Of course it'll make a difference if you remove the tip, you've shortened the total antenna length.



"And i take it this was wrong as well...."All a auto tune antenna does is fools the transceiver into thinking it has a matched antenna to transmit through, an auto tune does not act as a substitute for a right length antenna or increase performance."

That statement is correct, but there are many ways of making the antenna present close to 50 ohms to the transceiver - some radiate well, some don't. The reason the Codan has a reputation for good performance is because a lot of the tuning system becomes a radiating part of the antenna. There was an expensive broadband antenna sold years ago that was just a dipole with a 50 ohm resistor across the terminals !

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