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75 M G4ZU Jungle Job?




 
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K6JEK
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« on: November 08, 2018, 07:13:18 PM »

Any of you jokers know of a Jungle Job beam for 75 or 40? Living under a rock as I do, I hadn't even heard of this antenna until today. It's a two element beam with the driven element bent towards the straight reflector to form a triangle. It's a cousin of the Moxon rectangle, the hex beam, X beam etc. Folks build all of these for the higher HF bands but the Jungle Job looks like it might be pretty easy to string up as 75M beam. However, while the Moxon has been carefully analyzed by Cebik and others and  even has design website, I find nothing for the Jungle Job despite its superior name.

I know in the Moxon and hex beam, the tip spacing is critical. I would imagine that would be true here. Not incidentally, I've seen it drawn both ways, driven element bent or reflector bent. 


* Le Jungle Job.png (595.41 KB, 1226x819 - viewed 85 times.)
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K1JJ
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« Reply #1 on: November 08, 2018, 07:40:15 PM »

Hmmm..

Well, let's take one of the formulas that shows  150/f for the reflector.   Normally for a tapered tubing  (~1/2 wave *1.05) reflector element on 3.8 Mhz, the formula is closer to 515/f.   This means a 3.8 MHz reflector element needs to be about 140' long (full size) and will probably weigh about 100 pounds itself. BIG antenna.


I don't see any inductive loading to reduce its length. They are calling for a 40' reflector element on 75M.  [150/3.8]  Huh

There may be a little end loading due to the wires at the end of the reflector, but not much.


Something is wrong here....   It's fed with 50 ohm coax which is also way off being only 40' long with no matching method. The antenna as pictured would probably have an input feed impedance of 5-8 ohms max on 3.8 Mhz?  7:1 swr?   5dB forward gain on 3.8 MHz?  Not!

A 36' long reflector version (everything scaled) of this antenna would probably work on 20M. Model it and see.


BTW, I like the Moxon design and would recommend it.


T
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
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K6JEK
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« Reply #2 on: November 08, 2018, 07:55:43 PM »

Hmmm..

Well, let's take one of the formulas that shows  150/f for the reflector.   Normally for a tapered tubing  (~1/2 wave *1.05) reflector element on 3.8 Mhz, the formula is closer to 515/f.   This means a 3.8 MHz reflector element needs to be about 140' long (full size) and will probably weigh about 100 pounds itself. BIG antenna.


I don't see any inductive loading to reduce its length. They are calling for a 40' reflector element Huh?


There may be a little end loading due to the wires at the end of the reflector, but not much.


Something is wrong here....   It's fed with 50 ohm coax which is also way off being only 40' long with no matching method. The antenna as pictured would probably have an input feed impedance of 5-8 ohms max on 3.8 Mhz?  7:1 swr?   5dB forward gain on 3.8 MHz?  Not!

BTW, I like the Moxon design and would recommend it.


T
I've seen that same diagram in French and in Greek but not in English. Seems to be the thing and wrong. I've also seen it drawn the other way with the reflector bent. Everything I've read about Moxon rectangles impresses. I've seen but now can't find photos of a 75M Moxon. That impresses too.  The appeal of Jungle job is since the one tree died and had to be removed, I'm short a support. However, this thing could be strung up with three which I do still have.
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nq5t
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« Reply #3 on: November 08, 2018, 08:47:56 PM »

Hmmm..

Well, let's take one of the formulas that shows  150/f for the reflector.   Normally for a tapered tubing  (~1/2 wave *1.05) reflector element on 3.8 Mhz, the formula is closer to 515/f.   This means a 3.8 MHz reflector element needs to be about 140' long (full size) and will probably weigh about 100 pounds itself. BIG antenna.

The formulas in the graph give element lengths in METERS.  They are correct in that regards.  Either way, an 80M version of this antenna will be pretty good size ..

Grant NQ5T
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K1JJ
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« Reply #4 on: November 08, 2018, 08:52:14 PM »

Hmmm..

Well, let's take one of the formulas that shows  150/f for the reflector.   Normally for a tapered tubing  (~1/2 wave *1.05) reflector element on 3.8 Mhz, the formula is closer to 515/f.   This means a 3.8 MHz reflector element needs to be about 140' long (full size) and will probably weigh about 100 pounds itself. BIG antenna.

The formulas in the graph give element lengths in METERS.  They are correct in that regards.  Either way, an 80M version of this antenna will be pretty good size ..

Grant NQ5T


Now that makes sense!  A classic example of an American thinking the whole whirl is still English...   :-)


Yes, essentially this is a 2 element 1/2 wave parasitic beam and will be almost as big, but somewhat smaller due to the reflector end loading. (assuming the ref end leaders are both wire, not rope)   The advantage here is it is rotatable and self-supporting.... and not needing tubing for the driven element  Though tubing could be used for the driven el if supported well.

T
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
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« Reply #5 on: November 08, 2018, 10:32:48 PM »

That antenna appears in one of the ARRL compendium books I believe.  If not, it's in one of the other three or four antenna books I've got....

I did run across this:


http://forums.qrz.com/index.php?threads/jungle-job-antenna-g4zu.135296/


And here's an article by g4zu

http://www.cqham.ru/junglejob.html

Google translate will be your friend here it's translated to russkie.

--Shane
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K6JEK
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« Reply #6 on: November 08, 2018, 11:00:18 PM »

Thanks, guys. I especially liked the original article except in Russian. 
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W1ITT
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« Reply #7 on: November 08, 2018, 11:10:03 PM »

The trouble with horizontal parasitic antennas on 80 meters is that most people don't have enough of a tower to get them high enough.  Close to the ground, the mutual impedance with the ground image messes up the phase and amplitude relationships.
On 10 meters, or even on 20, the average joker can often get them high enough to perform pretty much as expected.
My two element 80m wire array is driven, so effects of mutuals are swamped out.  I suspect that the Jungle Job" got it's good reputation up at the top end of the HF spectrum.  
Back in the 60's, some W4 fellow was selling an antenna for 80 or 40m called the Guerilla.  It was a half wave dipole over a bunch of wires on the ground, directly under the driven element, the reflector wires laid on the ground in line with the dipole.  It was basically an NVIS antenna before that concept got to be cool.  The guy claimed "gain" of 10 db for close in work, but it was most likely not effective for DX.
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K1JJ
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« Reply #8 on: November 09, 2018, 12:21:36 AM »

Yep, under 1/4 wave high or less, an all driven array is the way to go.


The biggest project for high angle I ever did:

Some years back I built two single elements for 75M, aluminum tubing, each fed at the center as a dipole, 135' long each. They weighed about 100 pounds each, starting with 4" pipe at the center, tapering down to 1/2" at the ends.

I stacked one at 190' and the other at 60'.    I fed them in-phase and 180 out-of-phase, switchable from the shack.
As a reference antenna, I had up a flat 75M dipole at 65' located about 400' away from the stack.

I knew the stack was working well because in-phase DX signals from EU were loud and the locals dropped 20dB. And out-of-phase the locals came way up and DX dropped down.
On the computer modeling showed out-of-phase with the lobe straight up - beautiful NVIS.

After spending countless hours over several weeks receiving many signals local and DX on 75M, I was quite surprised. For local work nothing could beat the simple reference dipole at 65' high.  The stack out of phase was usually the same strength locally.   I eventually took the stack down and replaced it with the simple dipole and the uni-directional delta loops that I've used for about 10 years now.

The bottom line is that out of all the NVIS arrays I have tested on 75M (for local work out to 250 miles or so) nothing beats a simple dipole at 65'- 90'.   Optimum angles for 75M 100 miles or less are usually about 60-70 degrees take-off, so why bother?


BTW, I ran tests on a dipole at 30' using 1/2 wave reflectors under it. This antenna was ALWAYS down about 5-8 dB compared to the higher dipole for close in work.

Smile Boys - if you have up a FLAT 75M dipole 60'-90' high, (depending on your ground and terrain, and country's population density) it doesn't get any better than that for the locals.

T
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
K6JEK
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« Reply #9 on: November 10, 2018, 11:57:30 AM »

Yep, under 1/4 wave high or less, an all driven array is the way to go.


Yes, the best 75M antenna I ever had was a low, all driven array. However, an all important tree is now gone. Moxon rectangles and Jungle Job antennas claim to have close coupling between the elements courtesy of the nearness of the element tips. That sentence appears, more or less, in every write up of the Moxon rectangle, but there is no further commentary. I wondered if this might help mitigate the effects of being too darned low, just as driving both elements does.

If I had a whole lot space and supports, I'd love to compare a low Moxon to a low all-driven array, not in EZNEC but on the ground.
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