hf wire tap antenna

Submitted: Tuesday, Mar 30, 2010 at 20:26
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Can anyone tell me if you can mount a wire tap antenna behind the cab on a ute. This would mean that the bottom 700/800mm would be blocked by the cabin, will this have any affect on transmission or recieving signals. I don't really want to mount the antenna on my bull bar as I already have uhf, phone & gps antennas on it. Any advice would be great.

cheers Murray
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Reply By: SteveL - Tuesday, Mar 30, 2010 at 21:28

Tuesday, Mar 30, 2010 at 21:28
The radiation patterns will be severely affected by it's proximity to the bodywork of the car.Codan has done plenty of research on antenna placement and the bullbar mount works best.-Steve
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Reply By: Robin Miller - Tuesday, Mar 30, 2010 at 22:05

Tuesday, Mar 30, 2010 at 22:05
Hi Murray

It should have no measurable affect at all on receiving.

The effect on transmission is a little more complex , radiation efficentcy varies with current density and this is usually greater near the lower end where the tap and coil tends to be.

Even on a bullbar the coil can still be near the body of the car and affected.

When behind the ute cabin mount it as far as you can from body work and I would definately keep it 300mm + away, and as high as practical.

Most important is that the whip is then tuned and checked on the actual vehicle as opposed to a preset position - this will minimize problems.

Overall you can end up with a satisfactory result , even though it is not optimum.
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Follow Up By: SteveL - Tuesday, Mar 30, 2010 at 22:15

Tuesday, Mar 30, 2010 at 22:15
I don't like the chances of getting the antenna 300mm from the bodywork on the Mazda Bravo with the camper on the back.
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Follow Up By: Member - Murray R (VIC) - Tuesday, Mar 30, 2010 at 22:47

Tuesday, Mar 30, 2010 at 22:47
Hi Robin
Thanks for that info, my chances of getting the antenna 300mm from body is not practical, so it looks like bull bar mounting especially with the camper on as Steve has said.
Murray
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Follow Up By: Nomadic Navara - Wednesday, Mar 31, 2010 at 15:55

Wednesday, Mar 31, 2010 at 15:55
Quote - "It should have no measurable affect at all on receiving.

"The effect on transmission is a little more complex , radiation efficentcy varies with current density and this is usually greater near the lower end where the tap and coil tends to be."

The electrical characteristics of an antenna is reciprocal when it comes to the Tx and Tx. Or in other words, mounting the antenna in an unfavourable position will effect both the Rx and Tx the same.

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Follow Up By: Robin Miller - Wednesday, Mar 31, 2010 at 16:37

Wednesday, Mar 31, 2010 at 16:37
Hi Peter

I can understand how it might seem that way but it isn't as their is a fundamental difference and that is noise characteristics, these don't come into power transfer equations.

If an antenna is poor and transmits at 50% then the received signal is always 50%.

When receiving HF at the limit of communications the signal you hear is made up of two , not one component.

Its made up of signal + noise (manmade & atmospheric).

if your 50% effective antenna now reduces the received signal 50%
then both the noise and signal are reduced 50% but the ratio of the two remains the same.

Hence the ability to hear the signal remains essentially the same.

The above holds for HF but not for some other bands where the noise limitation comes from other sources e.g. device front end noise figure.
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Follow Up By: Nomadic Navara - Wednesday, Mar 31, 2010 at 22:36

Wednesday, Mar 31, 2010 at 22:36
Quote - "The above holds for HF but not for some other bands where the noise limitation comes from other sources e.g. device front end noise figure."

The front end noise figure is the limitation to the sensitivity of every receiver. However when you are in a noisy area the background noise can be a bit larger than receiver noise.

When you are looking at the signal to noise ratio of a transmitted signal, we will see that the signal to noise ratio maintains the same ratio for signals where the power level of the signal is in excess of the background noise power level. When the noise in the signal decreases to that of the background noise and the receiver front end noise the received signal to noise ratio commences to reduce and thus get worse.

If you reduce the efficiency of your antenna by mounting it in a bad position the signal to noise ratio commences to reduce much earlier than for wel mounted antenna. You will also loose the readability of the signal at an earlier stage than with a good antenna as the signal is fading.

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Follow Up By: Robin Miller - Thursday, Apr 01, 2010 at 07:56

Thursday, Apr 01, 2010 at 07:56
Meant to add that there is a simple test to verify that your receiving is not overly affected by antenna performance.

Simply listen to the receiver and connect/disconnect the antenna.

Almost always on HF you will hear an increase in background noise with antenna plugged in, and as long as you do the receiving is limited by that noise and not primarily the antenna.

It is because the external noise so dominates that your portable radios can get away with small highly inefficent antenna's.







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Reply By: _gmd_pps - Tuesday, Mar 30, 2010 at 22:33

Tuesday, Mar 30, 2010 at 22:33
you will have a reduction in radiation and depending on power output and wave length you might actually interfere with car electronics.

Bull bar above bonnet or the rear corners of the bed is a good place. if you
have a camper on the back like me than you may remove the antenna when
loading the camper and put it on the camper.

Of course it depends on size of the antenna. If you transmit with lower power (like 10-50 Watts) the ECU of the car is probably save. If you run an amp or use a TS-480HX you may want to look first where your ECU is under the bonnet an stay away from it.

A bullbar mount is good because the bonnet is acting as a ground plane and shields the car electronic a bit. Behind the cabin you should not have a problem with the ECU but you will affect your output.

Take the smaller antennas away from the bullbar and mount them on a Z-bracket. Why do you need a GPS antenna on the bullbar ?? My GPS's works inside the cabin without external antenna and I use a mouse GPS for the navigation PC and a Yaesu gps-1 mounted to a FT-8900 body for APRS.

good luck
gmd

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Follow Up By: Member - Murray R (VIC) - Tuesday, Mar 30, 2010 at 22:58

Tuesday, Mar 30, 2010 at 22:58
GMD
My Bravo is a old diesel so interferance should not be a problem. From what Robin & steve have said, it looks like bull bar mounting. As per GPS antenna I have 2 GPS of whch 1 requires an ext antenna. Do you know how close I can mount the HF antenna to other antennas re uhf phone without causing a problem.

Murray
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Follow Up By: _gmd_pps - Wednesday, Mar 31, 2010 at 01:26

Wednesday, Mar 31, 2010 at 01:26
again a matter of power output
mount receiving antennas on one side and the transmit antennas on the other side when you transmit with more than 10 watts.
If you have a transmit antenna right next to a receive antenna it will overload the receiver most likely.
good luck
gmd
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Follow Up By: Mike DiD - Wednesday, Mar 31, 2010 at 15:48

Wednesday, Mar 31, 2010 at 15:48
"A bullbar mount is good because the bonnet is acting as a ground plane"

- at a wavelength of 90 metres, a 1.5 metre long bonnet will be useless, anyway, there's no bonnet in front of the bullbar. If you don't have an inch-wide strap running from the base of the antenna to the front edge of the bonnet - forget it.
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Reply By: Bob of KAOS - Wednesday, Mar 31, 2010 at 07:33

Wednesday, Mar 31, 2010 at 07:33
Murray

You could mount it higher up (above roof height) by

a. attaching a mounting flange directly to the bodywork adjacent to the roof, or

b. mounting it on a single roof rack bar
AnswerID: 411149

Follow Up By: Nomadic Navara - Wednesday, Mar 31, 2010 at 20:53

Wednesday, Mar 31, 2010 at 20:53
Murray

I will go along with that. I take it that the vehicle you are mounting the antenna on is the one pictured in your signature. The question is, when you open up the camper on the back does it come forward over the bar between the camper and the cab? If not then I would be inclined to mount the antenna on that bar.

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