HF installation (Co-Ax)

Submitted: Sunday, May 06, 2007 at 08:48
ThreadID: 45141 Views:2453 Replies:8 FollowUps:12
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Morning all,
Should the Co-Ax from the transceiver to the antenna be as short as possible or should the cable be installed as issued by the manufacturer?

Thanks in advance.

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Reply By: samsgoneagain - Sunday, May 06, 2007 at 08:52

Sunday, May 06, 2007 at 08:52
i dont know about coax length. but i found that if you can run all your cables as far apart as possible ( at least 100mm) it seems to work better. i just seperated mine and it made a big difference.
AnswerID: 238176

Follow Up By: blue one - Sunday, May 06, 2007 at 09:19

Sunday, May 06, 2007 at 09:19
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Reply By: Member - Roachie (SA) - Sunday, May 06, 2007 at 09:22

Sunday, May 06, 2007 at 09:22
Not sure about other brands (eg: Codan), but with my Barrett, the cable from auto tune antenna is a 2-part/integrated cable. It is all wrapped up together and has the coax for the antenna as well as the "data"? cable with a small plug.

It would not be practical to shorten this cable IMHO. Even the extension cable you can get from Barrett, is an all-in-one unit.

So, my suggestion would be to leave the cables as supplied.
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Follow Up By: blue one - Sunday, May 06, 2007 at 09:50

Sunday, May 06, 2007 at 09:50
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Follow Up By: Member- Rox (WA) - Monday, May 07, 2007 at 21:43

Monday, May 07, 2007 at 21:43
Mine is a barrett as well with the extension aprox 3m. The excess is curled up under the rollor draws No problems.
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Reply By: Mike Harding - Sunday, May 06, 2007 at 10:57

Sunday, May 06, 2007 at 10:57
A very good question Blue.

Last week I trialled my new, home made, HF antenna. It's designed for fast set-up and take down and is a wire dipole cut for 80m (Amateur Radio). Now, I tune my antenna through a manual antenna tuner:

Site Link

which has proven to be an excellent unit over the past two years and will tune just about anything (including a wire fence - a shopping trolley is next on the list :) however try as I might I could not get the SWR down to a decent level. To cut a long story short it transpired the length of RG58 co-ax I had chosen, at random, as being suitable for camp use was some sub-multiple of the wavelength I was using and was causing the SWR problem. By adding a 15' extension length of co-ax I had brought along I was able to tune the antenna perfectly. The moral is don't shorten the cable unless you _really_ need to and appreciate that by doing so you may cause SWR problems. Do not coil excess cable (that creates RF issues) but rather "loose" it in sweeping loops etc.

Mike Harding
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Reply By: Tony Middleditch - Sunday, May 06, 2007 at 11:49

Sunday, May 06, 2007 at 11:49
Blue one.
Coaxial cable is known as an unbalanced transmission line. This is because the outer shield is earthed and therefore the RF (radio frequency) currents in the inner conductor and the shield are not equal.
The benefit of this type of transmission line (over balanced lines - TV ribbon) is that the performance of the cable is not adversely affected by length or proximity to other objects.
When using an unbalanced transmission line, the antenna should be of the correct resonant length and the antenna input impedance should closely match both the transmission line impedance and the output impedance of the radio (50 ohms unbalanced).
Coaxial cable performance drops off due to attenuation as the frequency increases. This is not really an issue for HF mobile installations. The attenuation will increase if the coaxial cable has high standing waves on it (SWR). For this reason, any antenna matching circuitry should be at the antenna end of the coaxial cable - as is the case for auto tune antennas.
RF feed back due to earthing deficiencies and stray RF pick up through other cabling can be a problem in a mobile installation. As a guide. Earth everything well with good sized earthing straps and use ferrite suppressors on other cables if required in order to suppress common mode currents.

Cheers, Tony.
AnswerID: 238197

Follow Up By: MichaelR - Sunday, May 06, 2007 at 21:03

Sunday, May 06, 2007 at 21:03
A couple of questions re your post. What exactly is a ferrite suppresor and what should they be wired to? (I have a Codan 9323). Where do you get them from. You mentioned earthing straps. Is there an advantage in those woven copper type cables as opposed to normal copper multistrand wire?
Thanks for your feedback
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Follow Up By: Grungle - Monday, May 07, 2007 at 08:26

Monday, May 07, 2007 at 08:26
A ferrite supressor is a ferrite tube or ring that you pass your cable through and loop around a second time to supress interference.

site link

You find them on such things as data cables (for cameras, pda's, scanners etc), AC power cables and video cables to name a few. Not all manufacturers use them but they are an effective low cost solution for preventing RF interference.

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Follow Up By: Tony Middleditch - Monday, May 07, 2007 at 13:37

Monday, May 07, 2007 at 13:37
Michael. David has partially answered your question re ferrite suppressors.
They can be made by using a soft iron toroidal core - as used in toroidal transformers or baluns.
Also common are the clamp on type that open up and the cable is placed in the middle of it.
I found the easiest and cheapest method to simply use some ferrite rod - as used in A.M radio antennas- and simply wrap the cable around the rod, using tape and cable ties to hold it in place. An example where this was required was with my IC706Mk2G (amateur radio) with the front panel remote mounted. The connection cable is 5 metres long which is 1/4 wave length on 20 metres (14mHz). Operating on 20 metres caused so much RF feed back that the radio would turn itself off.
Ferrites can be used to help stop radio interference where RF is generally being picked up form somewhere else than the antenna, example long speaker wires in your car or home connected to an amplifier / TV / radio etc.

Ferrite material used to be available from Dick Smith. You could try Jaycar,TTS systems etc.

Earth Straps.
RF is just AC (alternating current). It is just that the frequency is higher than the AC power at a power point. Voltage and current are 90 degrees out of phase. As the frequency increases, RF current tends to flow more on the outside of a conductor than through the middle of it. This is Known as the "Skin Effect". Thus earthing conductors with a greater surface area will be more effective than large solid conductors.
Earth straps should be kept as short as possible or else they may radiate stray RF. Antennas such as the Codan 9350 use just such an earthing strap.
In UHF base situations, a type of coax known as heliax is used this is low loss and uses a hollow copper tube as an inner conductor.
I have found that simple -short- earth wires OK for earthing things such as the remote head in my Codan 8528. No high RF currents should be present here anyway.
I hope that this helps, Cheers Tony - VK3CAT, VKS737 V5715.
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Follow Up By: MichaelR - Monday, May 07, 2007 at 22:14

Monday, May 07, 2007 at 22:14
Thanks Tony and Grungle. Very interesting and helpful information. Tony, you mentioned the earthing strap on the Codan which is as you have described. Presently I have it earthed to the chassis of the vehicle but was wondering if I should run a further (wide, flat) earth strap from the chassis to the body. Any thoughts?
Also do you use just one ferrite suppressor per cable irrespective of its length?
Thanks for your patience.
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Follow Up By: Tony Middleditch - Tuesday, May 08, 2007 at 12:18

Tuesday, May 08, 2007 at 12:18
Michael, if it all works OK - no RF problems or interference or tuning problems, then I wouldn't bother with the strap to the body. If you decide to do it though, do the strap as close to the antenna as possible.
One ferrite per cable is OK. Multiple ferrites will improve the de-coupling of stray RF common mode currents in the cables as will multiple turns through the same ferrite. For this reason, the cheaper ferrite rod can be best.

Probably should start another thread if this topic is to continue.
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Reply By: Footloose - Sunday, May 06, 2007 at 12:27

Sunday, May 06, 2007 at 12:27
With an auto tune there's no real reason to worry, unless the length causes tuning problems after the aerial is properly earthed. Keep the coax away from the remote control head lead as per the instructions and you'll be right.
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Reply By: blue one - Sunday, May 06, 2007 at 12:53

Sunday, May 06, 2007 at 12:53
Thanks to all great feedback.
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Follow Up By: Olcoolone (SA) - Sunday, May 06, 2007 at 13:17

Sunday, May 06, 2007 at 13:17
I take it the Codan radio arrived OK.

Regards Richard
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Follow Up By: blue one - Sunday, May 06, 2007 at 17:51

Sunday, May 06, 2007 at 17:51
Yes mate,
Spent all yesterday and today installing it. Wouldn't fit under the passenger seat so I made a bracket for where the kiddy seats use to be mounted in the back. I think I have done all the right things, seperation of the co-ax from power and control cable where possible, earthing everything that moves and even earthing the control head.

Just waiting for a rear mount bracket from SA, then I will give you a yell and see how she goes.

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Reply By: Member -Signman - Sunday, May 06, 2007 at 13:19

Sunday, May 06, 2007 at 13:19
Hey blue
Don't forget to run the power thru a HIGH VOLTAGE 20A fuse- not the normal 12v20A spade or glass type fuse.
AnswerID: 238210

Follow Up By: Mike Harding - Sunday, May 06, 2007 at 13:32

Sunday, May 06, 2007 at 13:32
Why do you suggest that?
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Follow Up By: Stu050 - Sunday, May 06, 2007 at 20:45

Sunday, May 06, 2007 at 20:45
It should be a cartridge type fuse.... Why 20 amps?

When I picked a cartridge fuse holder up for my 9323 from the local Codan Agent, a 32 amp fuse was supplied as standard?
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Reply By: blue one - Sunday, May 06, 2007 at 17:52

Sunday, May 06, 2007 at 17:52
Signman you mean a HRC fuse?

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