Quick ground plane question...

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Captain Fantastic
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Quick ground plane question...

Post by Captain Fantastic »

I want to use a 155cm tall spring mobile antenna as a base (like a Thunderpole Orbiter, not sure what make as I got it 25 years ago!), and would like to confirm my understanding before attempting it over the weekend...

The antenna is, as stated, 155cm tall with 11.5 coils over 15cm and 5cm diameter near the base, plus the extra 2.5cm (x2) to the middle of the coils.

I read that I need approx. 102 - 103 inch wires as a ground plane, attached to the antenna mount, and either 4 each at 90 degrees, or 2 at 180 degrees between them.

Is that correct? I am asking since I would use the antenna instead of a vertical wire (presumable of 102 inches as well) and the antenna does not seem to be 102 - 103 inches long physically but maybe the coils do something.

Thanks!
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wa10
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Re: Quick ground plane question...

Post by wa10 »

springers are loaded 1/4waves, all mobile cb antennas apart from full size 1/4wave whips are loaded 1/4waves electrically,

drooping the radials down like a starduster makes 1/4wave ground-plane antennas act more like a 1/2wave dipole & raises feedpoint impedance for a better match to 50ohm coax when using full size 1/4wave ( 102" ish ) conductors,

springers are one of the worst performing antennas available, you may get a decent vswr if you play around with radial angle & antenna/radial length,
don't expect starduster performance from a springer radiator,

good luck.
W8JI
"An antenna with a poor ground using few radials cannot have a support mast grounded to the radial common point (at least it shouldn't if designed properly) There is no exception to this!

Captain Fantastic
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Re: Quick ground plane question...

Post by Captain Fantastic »

wa10 wrote:
16 Jul 2020, 09:59
springers are one of the worst performing antennas available...
Good to know, thanks. I was potentially going to mount it on the car so now I won't bother and I'll stick with the Little Wil.
Also, I may now play around with three +/- 102" wires instead.
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Captain Fantastic
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Re: Quick ground plane question...

Post by Captain Fantastic »

OK so I had some time to play around, and I wanted to make a horizontal dipole antenna out of copper wire.
The coax goes into a wing mirror antenna mount, attached to a wooden pole stuck into a concrete base normally for holding your table umbrella!
Overall it is not very high, BUT I live at 630m above the sea with a large view, and this is placed on an upper terrace so is not technically near the ground... if that means anything.

Anyway having used the springer in and out of the configuration, I finally decided I wanted to avoid using it. I experimented with all sorts of wire lengths as recommended by calculators, etc., and measured SWR on EU 1, 19, 40, and the same on UK 1, 19, 40.

I started at 263cm on each wire and got SWR's about 3.0 on all channels.
For some reason I decided to lengthen the (what would be) 'lower' wire (connected to the outer coax) and with 274/263cm things improved to about 2.5 SWR. Maybe because a calculator had longer radials, who knows?
From there when I lengthened the 'upper' wire to 270 I was only impacting the UK SWRs back up to 3.0 so decided going higher on that end was not such a good idea.

So what I did was to extend the lower wire until I got good SWR on EU - this ended up being about 306cm.
Next I played with the upper wire again, and it came down to 245cm while having little impact on the EU but a big impact on UK SWRs.

Finally I've got about 1.5 - 1.7 SWR on all EU and UK channels which I am happy with for now (EU-ch01 = SWR 1.5 and UK-ch40 = SWR 1.7)

I also noticed that the total length is 306 + 245 = 551cm and this is about the 1/2 wave length I was thinking of for 27.185MHz when I set out. This could be a coincidence, or science. I'm hoping someone can tell me if I should have expected that?
My last shortening to 245 on the upper wire was kind of based on the idea it could be something to do with it...

For further info, my antenna is horizontally pointing almost at the middle of the UK from Zurich, Switzerland. Before I made these changes I was hearing guys from the north of UK and also Scotland. I expect to still hear them depending on the weather. Will they be able to hear me or do I need more power (I'm showing close to 4w on nearly all channels UK & EU). I am using a Cobra UK 25 STD ST from the early 90's.

Thanks!
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Stronty
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Re: Quick ground plane question...

Post by Stronty »

Making your own is always a little frustrating but feels good when it works out. Trying to understand things that influence the installation and the results if you're not getting what you read you should will inform you along with as much comparison as you can find.

might help, maybe, i'm not saying anything is correct on that link, just using it as one example of a useful calc. - http://www.ws6x.com/ant_calc.htm

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Re: Quick ground plane question...

Post by Captain Fantastic »

Are there issues I should be aware of with the two wires being at such different lengths? I'm just very surprised they are so far 'off' from what I read they should be. Maybe it's because there are other things close by.

Anyway, I'm hearing folks today, but not from UK - maybe conditions are not right or I was not listening at the right time of day.
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Re: Quick ground plane question...

Post by Captain Fantastic »

Quick update, and some success! I'm using the dipole mentioned above, with that stock Cobra UK 25 STD ST.

:o MY FIRST CONTACT!!!

This morning there seemed to be plenty of activity around. I could hear Italians, English, and German on the EU channels. I eventually plucked up some courage and called out to an English chap on UK ch33 after listening to him speaking to someone in Hampshire (I could not hear the Hants part). He was situated in the middle of France about 585km/365mi away and could just about hear me!
He reported I had 'mains hum' which I am not surprised about since I'm using a WiFi router 10v power supply rather than anything proper. I hear the hum myself on RX.

So, based on this milestone, my next thing is going to be a proper PSU. I'll check out the threads on here to see what I need.

Needless to say, I'm very chuffed. :thumbup:
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