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6/7/2018 10:09 AM
 

Among other things, I teach Antenna Theory. I've been working with the students the last several years to replace the roll up J-pole with a half wave dipole that you can easily walk around with (not typically easy to walk around the woods with a 3 foot antenna hanging off the transceiver). It outperforms all commercial antennas we've tested and may be field tuned and matched. The feedline uses a breakaway QMA connector in case of a fall. You can use the clips that came with the poles to make the other pole a parasitic radiator and, after retuning, you'll have a Yagi with about 4 dB more gain, as measured by the students last month. The only advantage to a roll-up J-pole is that it rolls up. You could certainly haul this pole up a tree and run well over 100W.

The link below points to part of a talk I gave at the APS meeting last week. I'll do a DIY youtube vid sometime.

Cheers!

https://drive.google.com/open?id=12D5...8sQanPvAmeg1CX9


 
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6/10/2018 2:59 PM
 

I just went outside and tuned one up. It will also tune to the Public Safety band at 155 MHz. I have uploaded a pdf that has construction notes, an exploded diagram, and a parts list.I have made three of these and each one is a little different in construction. The notes are just one way.

https://drive.google.com/open?id=1SxL...uKevUC74uDz7ofg


 
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6/12/2018 6:17 PM
 
I like this a lot. Is there a chance you'd sell an assembled kit?
 
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6/13/2018 8:36 AM
 
My intention is to sell the ones the students make for cost of materials so we could buy supplies for the next round of students. So far, they've only used Arco trimmers, which I will not let out into the public.Next year they will have some good antennas for sale.

I do have supplies for one more G3 Fixie -based antenna that I'd be happy to put together for you for cost of materials or if you have your own aluminum poles you want to hack up, that would work too.I'll need a few bits of information from you. Email is huskyrunnr(AT)msn.com
 
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6/17/2018 12:26 PM
 

The initial tests on these antennas ( in the APS talk) indicated that the MFJ Long Ranger, which should be as good as the ski pole, was down in power by almost 50%. Surprising. These tests were done at close range, 60 ft, with a directional wattmeter and a relative power slug. No batteries in the meter so this is a close range measurement. Yesterday, I decided to do a 10.3 mi. field test with the rubber duck, the Long Ranger, and the ski pole. Line of sight was poor. I was just going to use my ears to compare but I discovered a way to measure signal to noise without expensive equipment. It is probably publishable in a pedagogic journal. This is detailed in a pdf link below.

Bottom line, the rubber duck did not get in at all. Again, I get the MFJ is down by almost 50% power. The SWR is 2 and only accounts for 10% loss. I don't know if it is the matching section or the telescoping radiator or both that is so lossy. Weird, but at some level I don't care, the ski pole is much more pleasureable to use.

https://drive.google.com/open?id=11Sq...U6Ny07xie4KBMTS

 
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6/19/2018 8:43 AM
 
This is great! Thanks
 
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6/26/2018 6:13 PM
 

Thanks for reading along. This should just about wrap it up:

I haven’t said much about the two-element Yagi configuration of this antenna. The students were only able to get to 1.5 SWR. I don’t blame them, end fed antenna impedance cannot be modeled or measured without the matching network, as far as I can tell, so the antenna impedance is unknown but very high. The single fat ski pole dipole driven at the end is about 1500 to 2000 Ohms. We only know this after actual construction, matching and then tearing it down and measuring component values.

A center fed dipole has 72 Ohms impedance. If you add a parasitic radiator next to the driven dipole, and optimize for directional gain, you get a Yagi with a feedpoint impedance of about 2 Ohms. This is impossible to match over any reasonable bandwidth. Typically, designers do a couple of things. 1) They spread the elements apart to raise the two ohms closer to 50 Ohms, thereby sacrificing directional gain. 2) They use a folded dipole which by itself has 300 Ohm impedance and then drops to near 50 Ohms rather than 2 Ohms when the parasitic element is added.

What we have here, with the ski poles clipped together is an end fed Yagi. I’ve searched and this appears to be the first instance of this. In the same way a folded dipole has high impedance and makes it easier to design Yagis, so does an end fed dipole. Here is the matching network and the antenna circuit model. As I said, the students could only get to 1.5 SWR. Their C2 was the same as C1, 0.8 to 10 pF. I changed out their C2 for a 6-30 pF Arco trimmer,it will be replaced with a good piston trimmer. Both C1 and C2 were nearly maxxed out to get a match. Modeling this, we get that an end fed ski pole is about 1500 Ohms, but clip on the other pole (trimmed for best gain) and you get the input impedance of the end fed Yagi dropping to about 300 Ohms. The clips separate the elements by 1.5 inches. This is half the distance needed for best directional gain but we only sacrifice 1 dB of gain by using them. This matching network will still match the single pole.


The SWR was surprisingly flat. At first I thought I had constructed a 50 Ohm dummy load. I rebuilt the whole thing and got the same result.

 


NEC2 models predict 6.05 dBi, 15 dB front to back (F/B) gain ratio. The actual antenna is 5.7 dBi, 15.9 dB F/B ratio. This antenna will throw a signal a little over 1.5 times as far with the second element clipped in place.

 
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