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 It may be a daft question BUT.............

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BillMasen
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PostSubject: It may be a daft question BUT.............   It may be a daft question BUT............. Icon_minitimeThu Apr 23, 2020 8:55 am

I know why a vehicle antenna normally needs a ground plane like my van roof and the optimal length is 102 inches.

Sooooooo Why does a home base antenna have no need for a ground plane and needs to be nearly 18 ft long??


Which has me wondering if my 18footer from our old house can be cut down to 102 inches and fastened to the wall outside me den??

I really am a technophobe even though I've been on air since the 70s Smile
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NUBSTER
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NUBSTER

Call Sign : 26 CT050 26 FB080 26 WT105 26 TE171
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Join date : 2019-06-27
QTH or Location : PALMERS GREEN NORTH LONDON
Equipment Used : original President Lincoln, Midland 8001XT, President Teddy , Silver Eagle microphones , Gainmaster antenna, use a AMP ofcourse I do sometimes
Age : 62

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PostSubject: Re: It may be a daft question BUT.............   It may be a daft question BUT............. Icon_minitimeThu Apr 23, 2020 9:02 am

I'm like you Bill a technphobe lol, I know a bit but not all ,that question is a bit above me someone will come along and answer it ,hope in plain language lol
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BillMasen
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PostSubject: Re: It may be a daft question BUT.............   It may be a daft question BUT............. Icon_minitimeThu Apr 23, 2020 9:11 am

Cheers mate, I'm trying to get my Grant 2 up and running using what bits I have laying around here that I brought from our old house. EG 18ft antenna from old house but not allowed to put it up here, so thinking could I chop it down to 102 inches and bolt it to the wall Smile
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Gary1262
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PostSubject: Re: It may be a daft question BUT.............   It may be a daft question BUT............. Icon_minitimeThu Apr 23, 2020 10:03 am

Hi Bill,
You could try, but wouldn't recommend it.
I think there might be a coil plus other circuitry at the bottom of the 18 footer, which gets around the
need of a ground plane.
If you were to lob off a fair chunk of it AND even thought about using a ground plane, my guess is
it wouldn't SWR up.
What some people do, is make a vertical dipole, with the inner of your coax attached to the 102 inches going up vertical and another EXTRA 102 inches of wire going vertical down.
You might have to trim an end a little bit to get the coverted 1:1 SWR (plus tuner sometimes).
Problem with a wire vertical dipole, is that they are a pain to put up and support.
On the internet, you see some people making a horizontal dipole, with a connector block or 'proper' dipole centre piece to help keep the water out and can act as a centre piece support.
Horizontal antenna could also be better with skip! Very Happy
Fred in the Shed gave a good you tube video about a T2TL vertical antenna, which you can either make yourself or buy. This uses 102 inches vertical, but you'll have to watch the video for the rest of the construction.
Fred used an extendable fibre glass fishing pole as a support, said it was for temporary use, but I think might be ok permanent.
If you've got a tree, dangling from that could be ok.
Really need to get what ever you do, away from walls with this type of antenna, ground ok but higher the better.
Hope this helps, all the best,

Gary
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BillMasen
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PostSubject: Re: It may be a daft question BUT.............   It may be a daft question BUT............. Icon_minitimeThu Apr 23, 2020 10:13 am

Thanks gary.
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Richard.C
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PostSubject: Re: It may be a daft question BUT.............   It may be a daft question BUT............. Icon_minitimeThu Apr 23, 2020 7:58 pm

Your mobile is a 1/4 wave ground plane antenna and the 18 footer is likely an end fed half wave. Two completely different antenna designs.
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26TC62
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PostSubject: Re: It may be a daft question BUT.............   It may be a daft question BUT............. Icon_minitimeFri Apr 24, 2020 7:38 am

BillMasen wrote:
I know why a vehicle antenna normally needs a ground plane like my van roof and the optimal length is 102 inches.

Sooooooo Why does a home base antenna have no need for a ground plane and needs to be nearly 18 ft long??

Virtually all home base aerials benefit from some ground plane, even if they don't seem to need it to make an SWR match. Aerials which don't have ground planes either rely on being fixed to a metal pole (there's your ground) or tend to end up with the whole run of coax (specifically the outer sheath of it) becoming the ground, which in some situations can be a source of interference either outgoing or inbound.

You could use a length of tubing from your old aerial to make up a1/4 wave aerial but don't bother with whatever coil was at the bottom of it - and it will need a couple of ground radials or some metalwork for a ground to match properly.
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43CT016
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PostSubject: Re: It may be a daft question BUT.............   It may be a daft question BUT............. Icon_minitimeTue Jun 09, 2020 11:31 am

The need for a groundplane comes from the design. All quarter wave antennas need one, as do 5/8 wave. Half wave don’t, as long as the antenna design includes both “halves” of the antenna. This can be a traditional dipole, end fed dipole, or a half wave vertical with a matching section and using the coax, radials or other methods to “create” a ground.

This is an overly simplistic explanation, BTW. Very Happy

As for 102”, this isn’t necessarily optimum, it’s simply 1/4 wave on 11m and usually needs fine tuning depending on install and frequency.
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Northern Crusader
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PostSubject: Re: It may be a daft question BUT.............   It may be a daft question BUT............. Icon_minitimeThu Jun 11, 2020 4:12 am

43CT016 wrote:
The need for a groundplane comes from the design.  All quarter wave antennas need one, as do 5/8 wave.  Half wave don’t, as long as the antenna design includes both “halves” of the antenna.

Sorry but this is wrong. First of all, ALL end fed antennas need a RF ground/grounplane, even a half wave. It's because of physics, in particular Kirchoffs Law. Secondly the only antenna that includes "both halves" is a center fed antenna.

A bit of reading if you're bored.

https://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm
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43CT016
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PostSubject: Re: It may be a daft question BUT.............   It may be a daft question BUT............. Icon_minitimeThu Jun 11, 2020 9:47 am

Northern Crusader wrote:

Sorry but this is wrong. First of all, ALL end fed antennas need a RF ground/grounplane, even a half wave. It's because of physics, in particular Kirchoffs Law. Secondly the only antenna that includes "both halves" is a center fed antenna.

Other than my typo for end-fed dipole rather than centre-fed (vertical) dipole what bit don't you agree with, as that article says effectively the same thing...unless the antenna is balanced (i.e. has both halves) you need somewhere for the currents to go, which as I said in my post can be a counterpoise, radials, etc.?

Neither my EFHW or my old Stationmaster CB antenna (now on 10m) have a groundplane as such, but the EFHW is earthed at the transformer, and although I've never tested what difference it makes, has a choke 0.05 wavelengths away from the transformer, presumably to allow a part of the coax to run as a counterpoise, for example.

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