multi band ground plane -scanner

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srt1026

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i am thinking of creating a multi band ground plane antenna to be installed in my attic for a scanner. one element at approx 82.5 inches for 34ish, another element at 18.25 inches for 154ish, and the third at 6inches for 468ish. (photo 1) due to space limits, the ground plane will have to be at a 90 degree angle to the vertical elements.
question #1 would this work ??

question #2 measuring distance for veritcal elements (photo 2) from the ground plane up or just the element length ?? (gap between elements and ground plane would be about 1/2 inch)

question #3 would i need 3 separate ground planes, one for each band or will only one ground plane for the lowest band be ok ??

question #4 if i made the 154 and 468 elements collinear, would this effect the antenna at all--other than good :) ??
(photo 3)

question #5 does making these collinear have any effect on their harmonic performance ??

again, this would be in my attic so i can sturdy it up with some dowels or something. hope i made this so people can understand me, just learning my jargin by reading on internet.

any other comments or suggestions will be greatly appreciated


thanks all in advance
....steve
 

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djeplett

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Don't know if you got your idea from the discontinued Radio Shack one, but they made such an animal. I still have mine. Read about it here:
http://www.radioreference.com/forums/showthread.php?t=19006&highlight=20-014

So I can answer some of the easier questions:
#1) It should if you get the right resonance.
#2) The gap on mine is much more than 1/2"
#3) One ground plane, cut to the lowest band, will do just fine.
#4) Can't answer this one. My guess... probably.
#5) See #4

Hope I've helped somewhat. If you want, I can measure mine and give you dimensions... PM me.
 

srt1026

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thank you for the reply. if i may ask a few more specific questions, and i hope to explain them correctly :)

if i zoom in (see new photos), a 1/4wave element for 154mhz should be roughly 18inches long, now with the gap between the ground plane and where this element starts (B) being one inch, does that make the actual element (A) only roughly 17inches or does it stay at 18inches, (so A+B=18" or just A=18") ?

another question i have searched for an can not find a definate answer to.
the pasing stub of a collinear antenna(new photo), lets say for 154mhz, with a gap space of one inch(B) between the vertical elements, is the length of A and B both 18inches so the total "wire" length of the stub is 37inches (A+B+C), or is the total length of the stub 36inches (A+B+C) all the way around ?
 

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djeplett

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srt1026 said:
if i zoom in (see new photos), a 1/4wave element for 154mhz should be roughly 18inches long, now with the gap between the ground plane and where this element starts (B) being one inch, does that make the actual element (A) only roughly 17inches or does it stay at 18inches, (so A+B=18" or just A=18") ?
Attached is a closeup of the base of the 20-014 Radio Shack antenna...

The tape measure seems to show the insulator at the base is .75" but that's because I couldn't get the end of the tape down to the mounting base. It is almost exactly 1".
The distance between each vertical element is 2-1/8" and the element lengths are 16.5", 48", and the center element from tip to the insulator is 69". These measurements obviously don't work out to 1/4 waves at the center of VHF-Lo, VHF-Hi, and UHF, but I'd guess that they work out to 5/8 waves or more...

I believe the length should be just A=18"... Cut them longer and make them tunable so you can always shorten them up.

srt1026 said:
another question i have searched for an can not find a definate answer to.
the pasing stub of a collinear antenna(new photo), lets say for 154mhz, with a gap space of one inch(B) between the vertical elements, is the length of A and B both 18inches so the total "wire" length of the stub is 37inches (A+B+C), or is the total length of the stub 36inches (A+B+C) all the way around ?
From projects I've seen, it's the total wire length all the way around... so 36 inches if I read your question correctly.

Understand that I am not an antenna designer, nor do I want to come off as one. I speak from what I have seen online and from what I see when I look at my 20-014 which may not be a well designed antenna.
icon12.gif
 

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srt1026

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thanks for the timely response, that pic definatley helps that one question. i had one of those antennas at one time, and worked pretty well for me. but i had no clue on the exact measurements--thanks again.
the links u gave is one that i have been to before, and one that is screwing me up in my thinking. if u enter a freq of 156, it gives mesurements of (if u look at my previous pics) of A and C being 17.99 inches each, and then B being a gap of 1.63, giving the the whole wire (all the way around) a total of 37.61--1.63 inches longer than a 1/2wave all the way around. like u say, it may not matter but why not try to build it the right way.
thanks again for your input
 

djeplett

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You're welcome. Glad I could help with my limited knowledge.

srt1026 said:
if u enter a freq of 156, it gives mesurements of (if u look at my previous pics) of A and C being 17.99 inches each, and then B being a gap of 1.63, giving the the whole wire (all the way around) a total of 37.61--1.63 inches longer than a 1/2wave all the way around.
Evidently the gap is calculated with this in mind, to create the proper capacitance and I also think the element diameter affects the calculation as well. I think the larger the diameter of the element, the larger the gap needed, but don't quote me. (I'm thinking of a DC capacitor, so maybe this is not true?) Plus there may be a velocity factor being included in the calculation...

Maybe someone with a little more schooling or experience can weigh in? I want to learn something, too!
 

nd5y

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I'm not sure how they calculated the phasing stub gap but you might want to check out
Cushcraft's web site
http://www.cushcraft.com/amateur/support/instruction-manuals.htm
and download the instructions for these antennas:
ARX2B ARX-220B RINGO RANGER II ANTENNA
ARX450B ARX-450BN RINGO RANGER II ANTENNA

Unfortunately it doesn't give the width of the phasing stub but it does give the length for various
frequencies and as you can see the gap on the UHF ones is a lot narrower.
 
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It seems so obvious that someone needs to make a sturdy version of a tri-band outdoor antenna for scanner people. I actually had one of the RS things years back and cut the three pieces down to do 155, 460, and 860 mhz. It worked pretty well, but was stupid and left it at my old home. I drove by recently and it is still up, though I am sure not in use.
 

breadtrk

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I have dozens of old CB beams, MoonRaker 8's, JoGunn 6's, in my garage rafters being preserved by years of chicken poop. I reckon it is time to take them down and do some cuttin and cleanin. I looked at at my tower earlier and it is still standing after 40 years, I'll con one of the farm hands to climb up there and take down the full size home brew CB ground plane that is up there.

Damn RS and that 106 sale.
 

prcguy

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Looking at the original idea of a VHF-lo, VHF-hi and UHF whip over a ground plane, I see a potential problem. The 154MHz element will become a 3/4 wavelength element at 462MHz +/- 10MHz or so and a 3/4 wave whip has about the same impedance as a 1/4 wave whip. This will cause problems as similar current will flow in both whips on UHF and the impedance will be a mismatch to 50 ohm coax. I think it would work better if you just let the 1/4 wave VHF-hi whip act as a 3/4 wave on UHF (with its imperfect pattern) and maybe add a 3" whip for 800MHz.
prcguy
 
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Looking at the original idea of a VHF-lo, VHF-hi and UHF whip over a ground plane, I see a potential problem. The 154MHz element will become a 3/4 wavelength element at 462MHz +/- 10MHz or so and a 3/4 wave whip has about the same impedance as a 1/4 wave whip. This will cause problems as similar current will flow in both whips on UHF and the impedance will be a mismatch to 50 ohm coax. I think it would work better if you just let the 1/4 wave VHF-hi whip act as a 3/4 wave on UHF (with its imperfect pattern) and maybe add a 3" whip for 800MHz.
prcguy


I hadn't considered that, but you are correct. This is why the radio shack "sputnik" antenna works for VHF and UHF pretty well, 1/4 wave for VHF, 3/4 wave UHF, with the little 860 mhz stubs added for good measure. I guess I already have what I need after all.
 

thomast77

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I hadn't considered that, but you are correct. This is why the radio shack "sputnik" antenna works for VHF and UHF pretty well, 1/4 wave for VHF, 3/4 wave UHF, with the little 860 mhz stubs added for good measure. I guess I already have what I need after all.

I am looking to build a sputnik. I need a multiband antenna for around 155mhz/453-460mhz I have been looking for the measurements. Which I am having no success finding. So are you saying it is basically a 1/4 wave around 155mhz? 234/155mhz x 12=18.116" for the center radial? Why are there only 3 radials for the ground plane? And they appear to only slope around 20-30 degrees Instead of 45 degrees? If I have no use for 800mhz could I just build a 1/4 wave ground plane for 155mhz? Would it resonate anywhere around 453-460mhz?

I am also considering building a vertical bazooka for 155mhz using these plans.
2 Meter Vertical Bazooka - Build A Vertical Bazooka from Coaxial Cable by VE3VDC
Any idea if this will resonate anywhere near 453-460 mhz?

Also you mention 1/4 wave for vhf 3/4 wave for uhf. I cannot find a formula for 3/4 wave anywhere. Does anyone know the formula? Below is all I have been able to find.
Full wave = 936 divided by frequency.
5/8 wave = 585 divided by frequency.
1/2 wave = 468 divided by frequency.
1/4 wave = 234 divided by frequency.
1/8 wave = 117 divided by frequency
 
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