Author Topic: Prototyping the GR-P ¼-wave Groundplane Wire Antenna  (Read 5331 times)

Offline Greekman

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Prototyping the GR-P ¼-wave Groundplane Wire Antenna
« on: March 05, 2015, 03:24:12 AM »
GR-P as Greek Preparedness   ;D ;D ;D ;D ;D ;D

As title says, it is a ¼-wave groundplane for the VHF, made from wire with field expedient methods and tools. Its inspiration is the Jungle Antenna of the Vietnam era.

http://i176.photobucket.com/albums/w192/greekpreparedness/Fig7-13_zps1cjkctop.gif~original

Upon some further search the N6CC page was found, that has many more field expedient antenna designs.

So….Here she is.



Since wire is not stout, I chose to make a frame out of string tensioned by a wooden frame (just like the Jungle Antenna), then attach the wires on the string.

Construction

Starting with simple materials.



These are 4 pieces of 1mm speaker wire, a length of RG-58 coax, a electrical crimp connector , some zip ties and some heatshrink tubing.

First assembly step.



One can also strip the braid out of a length of the coax, and use the core as the vertical element. It saves the hassle of joining the wire to the core.
But 2 yards of coax was what I had available at the time, so I proceeded with adding the l/4 wire...



Half-done with a lot of reinforcement on the connection points.

The rest of the assembly was done in the field.



It is messy but it will all come to being clearer

Building the frame.



The radiators must be set at an angle of 45 degrees for the antenna to have an impedance of 50Ohms. So one slow night I sat down with a beer, a notebook and a scientific calculator and found out the frame and rigging dimensions.
I chose to have rigging strings longer than the wire, and decided on 60cm long. That dictates frame legs 73cm long.

And tie downs start to happen.



Finished up and hoisted.



The  wires were attached on the strings with a jumbled mess of electrician’s tape and zipties. But I will soon be heat shrinking the whole length.

And this is a posed view with element dimensions added.



Tuning



Notice the rookie mistake?

I kept removing 3mm (1/8”) at the time from the main element. When I called it quits these were the SWR measurements.



I was aiming for best SWR at the 145Mhz which is the lower limit of Greek HAM repeaters’ input freqs.
I missed it by some margin, not for overcutting, but for allowing ground effect to creep in. Antenna was kept at a bit over 1 meter from the ground to facilitate adjustments.

When I realised this I hoisted it above wavelength and took the final measurements above.



Adjunct

With the occasion I also setup my other ¼ groundplane, one made from single strand copper wire and a brass soldering rod (for the vertical element).
Here it is, posed on the ground.



I grabbed the chance to also “study” ground effect on tuning.
On the ground SWR was 1.37, 1meter over was 1.25 and raised above 2m SWR fell in the expected region of 1.05.
That was some lesson to be learned!
« Last Edit: March 05, 2015, 03:32:17 AM by GreekMan »

Offline Carl

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Re: Prototyping the GR-P ¼-wave Groundplane Wire Antenna
« Reply #1 on: March 05, 2015, 02:16:26 PM »
Reminds me of an indian DREAM CATCHER ...looks like fun.

Offline Alan Georges

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Re: Prototyping the GR-P ¼-wave Groundplane Wire Antenna
« Reply #2 on: March 05, 2015, 07:01:10 PM »
Good work GreekMan!  Thanks for the pics.

Offline Greekman

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Re: Prototyping the GR-P ¼-wave Groundplane Wire Antenna
« Reply #3 on: March 08, 2015, 10:49:26 AM »
i've got a question

Is there any reason why a vertical element 1/2 wave long would not work?

I am second guessing myself because of this Carl's reply
http://thesurvivalpodcast.com/forum/index.php?topic=32704.msg626246#msg626246

Offline Carl

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Re: Prototyping the GR-P ¼-wave Groundplane Wire Antenna
« Reply #4 on: March 08, 2015, 11:30:19 AM »
The half wave can be harder to get a good match due to inherently high impedance.
 There is little gain advantage.
The half wave works as an HT antenna due to little ground plane needed...but works best with NO FEEDLINE.
Some have used a half wave vertical ,folded back to where the top actually contacts the ground radials (so it is actually a quarter wave tall) so it acts as a shunt fed loop...this works OK and the shunt feed reduces static potential...but is of little advantage for FM.



Some reading at the link below.:

http://www.cvarc.org/new-wp/download/technical/antenna_myths.pdf


Offline Greekman

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Re: Prototyping the GR-P ¼-wave Groundplane Wire Antenna
« Reply #5 on: March 08, 2015, 12:27:41 PM »
inderesting read...had a look at some pages...

Quote
The half wave works as an HT antenna due to little ground plane needed...but works best with NO FEEDLINE.

it maybe be stupid but doesn't this scream 1/2 wave HT or Ft-817 antenna?






Offline Carl

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Re: Prototyping the GR-P ¼-wave Groundplane Wire Antenna
« Reply #6 on: March 08, 2015, 12:50:06 PM »
inderesting read...had a look at some pages...

it maybe be stupid but doesn't this scream 1/2 wave HT or Ft-817 antenna?

That is correct. A half wave whip is about as good as it gets for VHF HT/PORTABLE with no feedline.

Offline Greekman

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Re: Prototyping the GR-P ¼-wave Groundplane Wire Antenna
« Reply #7 on: March 08, 2015, 01:11:39 PM »
thanks Carl..if only my HTs had a BNC connector (scared to mount such a whip in the SMA ones)

BTW, Carl you do know you are mentoring me into more advanced HAM matters, bit by bit. Don't you?
TNX greekman

Offline Carl

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Re: Prototyping the GR-P ¼-wave Groundplane Wire Antenna
« Reply #8 on: March 08, 2015, 03:01:33 PM »
thanks Carl..if only my HTs had a BNC connector (scared to mount such a whip in the SMA ones)

BTW, Carl you do know you are mentoring me into more advanced HAM matters, bit by bit. Don't you?
TNX greekman

If I did it all at once ,you would resist.
You are one of many I offer bits of help to and ,so far,none resist.
My 'lessons ' are often way off of conventional teachings,though some smart people agree with me.
I do my best to tell what works and what I find to be correct even though not conventional wisdom.
You also have prompted me to learn some things. :)

Offline Greekman

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Re: Prototyping the GR-P ¼-wave Groundplane Wire Antenna
« Reply #9 on: March 09, 2015, 07:43:13 AM »
Quote
You also have prompted me to learn some things'
Really?

Quote
If I did it all at once ,you would resist.
Anyway I am a slow reader... I have no tiem toread on the PC and I just print inetrestign stuff to read on my leisure.
Problem is thta the last few years I doze off within a page or two..Maybe a sign on middle age?

Quote
My 'lessons ' are often way off of conventional teachings,though some smart people agree with me.
I do my best to tell what works and what I find to be correct even though not conventional wisdom.

well, it took me a while to read all the articles from the "Antenna and feedlines" link you posted.
I admit my head was turning when reading the last 2 chapters and i have kept the printing to re-read...