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Author Topic: End fed transmitting antenna with 9:1 UnUn.  (Read 15163 times)
AD5VM
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Posts: 26




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« on: August 10, 2011, 07:31:37 AM »

I have a question regarding the use of a 9:1 UnUn. I've searched the forums and maybe I missed it but I didn't find anything relating to my application.

I need to erect an end fed wire antenna for a temporary (a few months) application. I can put up a 100+ foot sloping wire with a feed point height of about 18 feet at the roof peak and end point height of about 60 feet in a nearby tree. I must use coax (unfortunately) as the feedline. (the roof is metal so this could be used as a ground plane)
The coax will be 20 foot of RG8X and if a transmatch is needed at the rig end, I have an MFJ-901B. (Rig is an IC-7000)

I would like to have multiband capability so I found this 9:1 UnUn at balundesigns.com

http://www.balundesigns.com/servlet/the-102/QRP-9-cln-1-Unun-1.5/Detail

He lists recommended wire lengths in this document:
http://www.balundesigns.com/Wire%20Length%20for%209132s.pdf

You can see the magic numbers of 52.5 or 124 feet show an SWR of well under 2:1 on ALL bands.  I plan to use 124 feet for my antenna.
It's seems this type of antenna is only ever used for receiving, but this UnUn is rated at 300 watts and he makes a 2KW version!
My question is simply, what's the catch? Why is this not a widely used multi band antenna? An end fed wire with a ~1.5:1 SWR on all bands, no traps, no resistors, just a simple matching transformer?? It seems too good to be true and I'm sure you guys will confirm my suspicion but I have to try it.
If this doesn't work as advertised I'll install my AH-4 coupler but I was really hoping I wouldn't have to (I use it for portable ops so it's currently mounted to a spiderbeam mast)

Thanks in advance!
Larry AD5VM.
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W8JI
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« Reply #1 on: August 10, 2011, 08:34:04 AM »

You can see the magic numbers of 52.5 or 124 feet show an SWR of well under 2:1 on ALL bands.  I plan to use 124 feet for my antenna.
It's seems this type of antenna is only ever used for receiving, but this UnUn is rated at 300 watts and he makes a 2KW version!
My question is simply, what's the catch? Why is this not a widely used multi band antenna? An end fed wire with a ~1.5:1 SWR on all bands, no traps, no resistors, just a simple matching transformer?? It seems too good to be true and I'm sure you guys will confirm my suspicion but I have to try it.
If this doesn't work as advertised I'll install my AH-4 coupler but I was really hoping I wouldn't have to (I use it for portable ops so it's currently mounted to a spiderbeam mast)

Thanks in advance!
Larry AD5VM.


I just ran a model of a 124 foot long wire sloped at a 45 degree angle, and the SWR with a LOSSLESS 9:1 transformer was never below 4:1 on any band.  Lowest SWR was 20:1 on 160 meters, 10:1 on 80, 7:1 on 40, and 5:1 on 20 meters.

SWR was typically about 6:1 above 40 meters, and climbed towards 28:1 on the low end of 160.


I think what you will find is that behind every bit of this recent magical "one wire works all bands with a low SWR" junk science being fed to consumers, is a great deal of system loss. If something was that good from the start, it would not have just appeared in the past several years.

Now it is true that a higher impedance feed system can reduce SWR excursions, but if it starts getting much flatter than 5:1 or 10:1 on peaks, you can pretty much be assured it is because of losses.

For example a ladder line fed dipole has a feed impedance varying between maybe 50 ohms and 5000 ohms. If I fed that dipole with 500 ohm line, SWR would vary around 10:1. If I started to see less than that SWR on lower bands, it would pretty much only be because of losses.

The oddest thing about this magical "one wire does all bands without traps" stuff that is creeping into our hobby is people won't use traps that might add 1 dB or so of loss down to no measurable loss at all.....but they will throw away 10 dB of loss without anyone saying a word. Traps have gotten such a bashing that people would rather have ten times the loss.

73 Tom
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AC5UP
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« Reply #2 on: August 10, 2011, 09:25:23 AM »

It's seems this type of antenna is only ever used for receiving, but this UnUn is rated at 300 watts and he makes a 2KW version!
My question is simply, what's the catch?

I wrote about this here http://www.eham.net/ehamforum/smf/index.php/topic,76728.msg530405.html#msg530405

In a nutshell I tinkered with terminated longwires a few years ago. The unun typically sold (and the one you're considering) for a 9:1 transformation is essentially an RF autotransformer with a limited frequency response. Works like a champ below 2 MHz or so then gets progressively more lossy until you notice the RX is starting to go deaf around 5 MHz.

If I was doing this today I'd likely use a dual-core Guanella 4:1 balun at the feedpoint, make sure I had a very good ground system, then see what the autotuner in the rig could do with the SWR bumps. Not a textbook perfect solution but the 9:1 unun option is only good through 160 meters. Although, it would make a dandy antenna for your crystal set...........
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W8JI
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« Reply #3 on: August 10, 2011, 09:57:32 AM »

The feed impedance of a lossless 1/4 wave antenna is about 30 ohms or so.

If we use a 9:1 transformer, SWR would be 15:1 on 160 meters.

If SWR is not at least 10:1 on 160, it has to be lossy as heck.

Things get better above 40 meters where SWR should be nominally are 5:1. Sensitivity and transmit signal should be much WORSE on 160 and gradually get better with increasing frequency.

If you go high enough in frequency so the wire is several wavelengths long, SWR should get pretty flat because the wire self-terminates.

Low SWR and better performance on lower bands indicates a serious system flaw.
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K0ZN
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« Reply #4 on: August 10, 2011, 03:33:04 PM »

Hi, Larry....

Tom, W8JI, nailed it.   To Wit:  Only a dummy load has good SWR on ALL frequencies !

My two cents would be to simply end feed the wire at the tuner and do everything you can to make a good RF counterpoise for it.
If you "have" to use coax between the antenna and the tuner, use large diameter, high quality coax. With very high SWR, the losses
in the coax will be significant, even a modest run of it will be lossy. If you take a single wire through a window frame, be sure to insulate
it very well.

When I was in college I operated out of a second floor apartment with long chunk of wire out the window with direct end feed, because there
was no way I could do anything else. That wire was a couple of hundred feet long and actually loaded and worked pretty well. The key was that I
grounded the tuner chassis to everything metal that I could see (gutters, window frames, wall socket "ground", water pipes, etc., so there was
a reasonably decent counterpoise. I am not saying this is the best way to do it, but being a young and slightly crazy ham at the time, I went that route.
Interestingly, I never had any TVI reports (that I knew of)!!

73,  K0ZN

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AA4HA
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Posts: 1398




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« Reply #5 on: August 10, 2011, 07:34:13 PM »

A long wire antenna that works with a 9:1 balun can also be a Beverage antenna. They are pretty low efficiency but when terminated with a non-inductive resistor they are very directional when longer than a wavelength. There are some Beverage antennas that can be bi-directional (off the end of the wire). Normally you mount them fairly low to the ground (6-10 feet, so you do not electrocute a deer or clothesline yourself when on the riding mower).

Finding a high enough power rated resistor would be a real challenge. This is why Beverage antennas are usually for receive only applications.

You could also try a Zepp or "double Zepp" antenna (reference at   http://www.iw5edi.com/ham-radio/428/double-extended-zepp-antenna  ). You will need to feed it with ladder line and a good antenna tuner.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
N4JTE
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Posts: 1155




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« Reply #6 on: August 10, 2011, 08:14:24 PM »

Have to agree with previous posts, nothing wrong with some relatively low loss traps as opposed to some 9 to1 toaster, sorry meant balun. End fed wires are tricky enough to work efficently as a monobander let alone multiband. My advice is to save your money on the toaster idea and use  temporary space for something more efficent. Hard to design from here but limited to coax  sloping fan dipoles comes to mind.
Good luck,
Bob
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PA3ALF
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« Reply #7 on: August 11, 2011, 03:57:31 AM »

Hello Larry

I use  an end fed at home in this configuration:
Rig is ft950 with autotuner, 17 meters of coax, a home made 1:9 balun  and a 16,2 meter wire going from the shack in the top of the house sloping down to 2 meters in the end of the garden. And it WORKS with the internal tuner on 80 40 and 20. I only use it on 80 because my garden is to small for a 2*20 meter dipole HI. I agree with the others however that the system has a lot of loss but it is better than nothing at all on 80. I Also used this antenna on holidays for my FT817. Mostly I get better reports when I use  a portable whip antenna. Other problem is that it is very noisy. On 40 m it has 3 s points more noise on receiving than on my dipole.  But if it is the only way for you  to put up a temporarily antenna  give it a try it.

Best regards
Jos, PA3ALF
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AD5VM
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Posts: 26




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« Reply #8 on: August 11, 2011, 07:03:03 AM »

Thank you everyone for your replies! I'm familiar with beverage antennas and balanced feeders, I've never had an antenna with traps. This could work, I could Put up an 80 meter 1/4w with a trap for 40 and 20. and feed it against the metal roof of the building.

Somebody mentioned the all mighty fan dipole.. I can't center feed this antenna but maybe I could put up a fan.... uni-pole fed against the roof, something like an alpha delta DX-CC cut in half.

I've still got the UnUn coming in the mail so while I'm waiting for the traps to arrive, I may try the 124 foot wire and just sweep it and report my results.

Thanks again.
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W8JI
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« Reply #9 on: August 11, 2011, 07:09:17 AM »

The original question here was whether a 9:1 transformer feeding a 52.5 or 124 foot will have less than 2:1 SWR on all bands as claimed.

On 160 the feed impedance of either wire is so low that a lossless 9:1 transformer would have over 20:1 SWR. On higher bands, it would be 10:1 or so gradually moving to around 5:1 at upper HF and eventually up at upper VHF to less than 2:1.

A system that has 2:1 SWR or less across HF requires roughly about 6 dB or more total loss in the system on several bands. Somehow somewhere in the system on several bands it will lose over 75% of the power as heat.
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W4VR
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« Reply #10 on: August 11, 2011, 12:15:39 PM »

When I got my novice license the first antenna I put up was an end-fed half wave wire on 80 meters up about 35 feet.  I fed it with open wire line and no balun.  It actually worked better than my Gotham V80 vertical.  Then, I got smart and center fed the wire...what a difference in signal reports.
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N2EY
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« Reply #11 on: August 12, 2011, 03:33:14 PM »

I could Put up an 80 meter 1/4w with a trap for 40 and 20. and feed it against the metal roof of the building.

You may not even need the 20 meter trap.

I have made and used several 80/40 trap dipoles, and the SWR on 20 is only 3:1 or so. If the feedline isn't too lossy and the rig can handle the SWR, you're set. It's not a beam but it's no worse than a G5RV or purpose-cut dipole.

<i>I've still got the UnUn coming in the mail so while I'm waiting for the traps to arrive, I may try the 124 foot wire and just sweep it and report my results.

I would return the unun unused if that's possible. Here's why:

As W8JI points out, the ONLY way the described end-fed antenna can have a low SWR on "all bands" is if there is considerable loss in the system. A lossy feed system (coax, unun, ground, etc.) can make *any* antenna's SWR look good at the shack-end.

The big problems with evaluating "magic" HF antenna performance only from user reports are two:

1) HF has high "dynamic range", in the sense that, when conditions are right, you can work the world with only a few watts of radiated RF. Which means even a terrible HF antenna can make SOME contacts if conditions are good enough.

This is good news for those of us with limited resources. In fact, it's what makes HF so much fun even if you don't have a great setup. But it can really lead to false conclusions about how well a particular antenna really works.

2) Few hams with limited space have the resources to do real-time A-B comparisons; HF antennas are just too big. This leads to the "compared to what?" situation:

Ham A: "I put up an XYZ antenna and it works GREAT!"

Ham B: "Compared to what?"

73 de Jim, N2EY
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G3RZP
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« Reply #12 on: August 14, 2011, 10:13:42 AM »

One effective end fed (but needs a tuner!) is the old W3EDP. Why it is so effective is a puzzle, but I found it far better than my expextations.
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