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Author Topic: Radial Placement  (Read 1528 times)
VA6TPR
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Posts: 2




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« on: September 29, 2004, 09:47:24 AM »

I am just starting to equip my 4x4 with my radios.  I have a CB and a quad bander - 10, 5, 2, 0.6M Yaesu.  Problem is, the removable roof on my truck is fibreglass... no decent ground plane.

So, I started with my CB install (rather burn out that transmitter than in the Yaesu!), and ran four 292cm long radials (I found a calculator on the web to find ideal 1/4 wave radials for a particular frequency.. this is tuned to CB channel 4, which is what I usually run on CB) with a 5/8 antenna.  

Now, it's just not practical to run a radial 292 cm straight out in a vehicle!!  So, I zig zagged the radials across the roof, carefull not to make a loop anywhere. The SWR is terrible.  On the 4Watt CB, I am getting only about .5W out, and almost 5W reflected back.  Tested the transmission line, and everything seems fine.  

Is there any hope to run radials inside a truck, or do I need to stick some sheet metal under the headliner to act as my ground plane?  Or do I just need to abandon my hopes for ideal placement on the center of the roof, and go with a lip mount next to the hood?

Peter Straub
VA6TPR
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VA3WXM
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Posts: 277




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« Reply #1 on: September 29, 2004, 12:00:27 PM »

I assume there's no short circuit in your feedline or current antenna mount.

If you can't tap into any vehicle metal for counterpoise/groundplane purposes you're antenna just won't work.  There are other mounting possibilities for the CB whip: ball mount on the rear quarterpanel or spare tire mount.  If that's not an option I believe there are "no-groundplane" CB antennas available but I don't know how well they work.

I'd save the hood lip mount for the FT-8900 and get a good tri-band antenna (or quad-band if you intend to use 10m much).
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WB2WIK
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Posts: 21837




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« Reply #2 on: September 29, 2004, 12:09:27 PM »

The radials should probably really be 264cm long each...292cm sounds a bit long for 27 MHz.  However, since they're not run in a straight line, it's a bit difficult to predict what length they really should be.

In any case, it's impossible to have more reflected power than forward power (you said "0.5W out, and 5W reflected").  If you're really reading that, there's something wrong with your SWR meter.

WB2WIK/6
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VA6TPR
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Posts: 2




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« Reply #3 on: September 30, 2004, 07:47:45 AM »

Yup.  There was something wrong with the SWR meter.  In the IT world, we would call it an ID-10-T error.  Or a PEBCAK.  

I had the input/output lines on the SWR meter reversed.  ARG!  I spent four evenings trying to figure out what was wrong.  Turns out that nothing was wrong, and my SWR was almost perfect with almost no power reflected back at all!  

I am so embarasssed.  

So, the lesson here is, zig-zagging radials around the roof does work.  

For the archives, I used the following pattern to run the radials:

From the center of the roof, I extended to one corner, then came back to the center, then out to the next corner 90 degrees away, then the remnant went along the roofline.  I used 18 Gauge wire for the radials.  I used 292cm radials, as some java app I found on the net told me that these were the correct 1/4 length radials to use for a 5/8 antenna.  I am getting excellent reception and the SWR meter tells me I am broadcasting just fine.

Peter Straub
VA6TPR
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WB6BYU
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Posts: 18518




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« Reply #4 on: October 20, 2004, 01:01:07 PM »

But you probably aren't using a 5/8 wave whip on CB
on the roof of your vehicle - that would be about 7m
tall!

As Steve (WIK) said, bending a wire around changes the
resonant length, so it becomes more an approximation
followed by an experiment than an exact calculation.
You could try trimming the end of the radial wire to
adjust the SWR, but it sounds like it is close enough
as it is.

However, if you are going to do the same thing with the
VHF antenna, you probably will be better off using some
metal sheet on the underside of the roof.  Look for
aluminium flashing in the roofing department of your
local building supply store as one possibility.

Doing this may disturb the tuning of the CB radial
however.  What may work better on all bands would be to
put a couple pieces of flashing under the roof for the
VHF bands, then tie the corners of the flashing to the
chassis/frame with  wires (wide strips of flashing would
be better) running down each doorpost or other support.
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