HF Tapped Whip on the Back of a Rangie?

[Davo]

I've got a feeling this won't work since the antenna may be too close to the body, but I don't know enough to say for sure. Pardon the dorky MS Paint sketch.

It would be mounted on the spare wheel carrier. I want to move it because when it's on the bullbar it tends to get in the way when parking under the carport or trees, so a lot of the time I just don't have it on the car, which then defeats the purpose of having the radio in the first place. I would try it behind the spare instead of next to it, but then it would be in the way of the top tailgate!

Many thanks, as always, for any help.

[patto1]

Hi Davo,
For what it is worth .... my HF antenna is mounted on the spare tyre ..... works well ... just put the base as high as you can .... see attached thumbnail

[wrinklearthur]

Hi Davo, my HF antenna is mounted on the spare tyre ..... works well ... just put the base as high as you can
The tip of those types aerials is a high voltage point, and the capacitive coupling from that point, is best kept to a minimum by keeping the tip as far away from the body as possible.

Good grounding straps from the aerial base to the chassis does help the performance of aerial, as the base is the high current point.


I've got a feeling this won't work since the antenna may be too close to the body. If properly mounted and the grounding is correct, it should work ok. What is the VSWR for that aerial ?


It would be mounted on the spare wheel carrier. I want to move it because when it's on the bullbar it tends to get in the way when parking under the carport or trees, so a lot of the time I just don't have it on the car, which then defeats the purpose of having the radio in the first place. I would try it behind the spare instead of next to it, but then it would be in the way of the top tailgate!

Many thanks, as always, for any help.

[Lotz-A-Landies]

Unlike many modern CB antenna, the multi-taps are ground plane dependant.

On some brands you will find a band about 12" to 18" from the bottom. This band marked the electrical ground plane and was intended to be aligned with the level of the bonnet, boot lid or roof.

If you have one of these types, try to align the band with the edge of your roof.

[Davo]

Thanks very much. I had to look up "capacitive coupling" and got the general idea, i.e., keep the tip of the antenna as far away from the car as possible because of, er, some reason to do with varying magnetic fields, but I get the gist of it.

I do have a little SWR meter that can handle up to 1000 watts, but while it will work on the old 27mHz radio in the car, the needle won't move with the HF. I'll try the other channels later on.

But following the general principle of the thing, I'll just make a frame to go onto the wheel carrier, get the antenna up as high as is practical with good earthing, and go from there.

[wrinklearthur]

Thanks very much. I had to look up "capacitive coupling" and got the general idea, i.e., keep the tip of the antenna as far away from the car as possible because of, er, some reason to do with varying magnetic fields, but I get the gist of it. That's good

I do have a little SWR meter that can handle up to 1000 watts, but while it will work on the old 27mHz radio in the car, the needle won't move with the HF. I'll try the other channels later on. Circuits specifically designed for the 27 MHz CB band don't work at the RFDS 2 MHz frequencies, you must use a VSWR meter that is made and labelled accordantly. see this example .
http://www.mcp.com.au/xcom760/manuals/XCOM-Avionics-SWR-and-power-meter-operating-instructions.pdf
When tuning your aerial, to prevent damage to the final output transistors of the transceiver and not cause any unnecessary interference to other users of the test frequency, tests must be done at the lowest transmit power possible.

But following the general principle of the thing, I'll just make a frame to go onto the wheel carrier, get the antenna up as high as is practical with good earthing, and go from there. Keep us informed.
...

[patto1]

Hi Davo,
You asked how the antenna base is earthed. The bracket that holds the whip base goes thrrough the centre of the spare wheel and is welded to a plate with holes the same as the wheel stud pattern. The spare wheel ... when bolted to the spare wheel bracket ,,,, sandwiches the plate with the holes ... earthing is via the spare wheel bracket (if that makes sense?) Works very well.

regards,

[wrinklearthur]

You asked how the antenna base is earthed. The bracket -----and is welded to ---------bolted to ---- the plate------ earthing -- via the spare wheel bracket ---

It may be a small improvement but I would do the following anyway, because of the number of joints and the hinge.

A simple braided earthing strap attached to the aerial bracket, down around the back of the hinge point and bolted as close as possible to the aerial onto the chassis, will stop a lot of static noise across the hinge, especially when the vehicle is moving while travelling.

It will do little in improving any VSWR mismatch at that point, as that is more to do with aerial design and it's placement. If fitted, the Antenna Tuning Unit (ATU) should take care of any loading problems and bring the VSWR down close to a 1:1 match for the transceiver, in reality for those types of aerial set ups, the VSWR for the frequency band edges are somewhere about 1:5 and the centre of the band about 1:3, I would be ok with those readings if it was my antenna setup.

With a few types of ground independent ATU's, watch out if the casing is electrically isolated from the mounting bracket, then the coax shield is the only ground permitted in that instance, but still use a grounding strap to the chassis and where the ATU is bolted to the frame.

Another source of RFI noise is where there is a corroded joint, this can act as a semiconductor junction. Cleaning the joint, then using a conductive jointing paste, while reassembling, usually fixes that one when you find it.
.

[Davo]

Silly me, I thought that little meter was okay at first. That one you put the link on for is about $230 from here, UHF/VHF/HF VSWR/POWER Meter 1.8 - 525MHz (http://www.mbradio.com.au/onlinestore/index.php?route=product/product&product_id=589) , so not too bad for something that useful.

One good thing about VKS-737 is that they don't mind radio checks at all, and this time of year is very quiet during skeds anyway.

Patto, I'm surprised that works. I would have automatically put the usual braided copper strap from the antenna to the chassis. Are you sure that's not your trouble? And I forgot to ask before: is your radio connected directly to the battery?

[Davo]

Keep us informed.

Yes, sir!

Well, I finally got it done. It was going to go onto the spare wheel carrier, but that rattles around too much, so I wound up making a jerry can holder since I was going to anyway, and the antenna bracket goes onto that.

http://www.aulro.com/afvb/attachment.php?attachmentid=48007&stc=1&d=1339922365

It's also designed so that I can loosen two bolts and lower the antenna quickly, which should help with trees around here.

http://www.aulro.com/afvb/attachment.php?attachmentid=48011&stc=1&d=1339922365

Testing shows that it works about as well as on the front. It's also a bit higher, and more importantly, looks cool! :p