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Author Topic: Shortened 40 meter Hexbeam  (Read 3470 times)
PD2R
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Posts: 131




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« on: March 22, 2013, 04:23:59 AM »

I have a 6 band (6-20 meters) hexbeam from Waldi SP7IDX. The antenna is very nicely made and I'm planning to put it on the roof this spring.
Up until now I have been using a 20 meter long end-fed wire (PAR eletronics) for the 40 meter band. It slopes from the roof of my house down to the back of my back yard, Highest point is 11 meters (36') of the ground, lowest point just 2,5 meters (8'2") of the ground. So basically this is an NVIS antenna.
The antenna has served me well, actually better then I expected.
Despite my good results with the end fed wire, I'm thinking of trying something else.

Searching the internet I've found the following shortened 40 meter elements for a standard hexbeam: PDF for shortened 40 meter hexbeam

My question is, will this antenne outperform my end fed wire? Now, I know the shortened 40 meter hex is a BIG compremise antenna since it's elements are only half size and folded into a W shape. It will probably be outperformed by the low wire for the short distance contacts but is it possible to predict how it will perform on longer distance contacts?

The proof of the pudding is in the eating but I plan to put the Hexbeam on top of my roof and leave it their for at least the next 5 to 10 years.
I'm not afraid of heights but climbing on top of a 45 degree angled, 10 meters (32') high roof is not something I would like to do for a hobby so I would like to do it right the first time.
Furthermore, Building the shortened 40 meter hex with quality materials (materials that will last) will also cost around €150,-.
So I'd like to know as much as possible before jumping into this experiment.

Looking forward to hearing your opinions.

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W5DXP
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« Reply #1 on: March 22, 2013, 05:22:03 AM »

PDF for shortened 40 meter hexbeam

Looking forward to hearing your opinions.

That antenna has a gain of 6 dBi and a F/B ratio of 1.4 dB (1/4 of an S-unit). I doubt it qualifies as a "beam" and one probably cannot detect the operational difference between it and a simple 1/2WL dipole. One might as well eliminate the reflector because it is doing hardly anything detectable at the receiving end. The only advantage I see over an ordinary dipole is that it is rotatable. My opinion is that it is not worth the effort but changing the length of the reflector might help.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
PD2R
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Posts: 131




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« Reply #2 on: March 22, 2013, 11:19:08 PM »

Hi Cecil,

Thank you for your reply. I actually wouldn't mind if it would do just as well as a rotatable dipole. I never expected "beam" performance. I regularly work with a 4 el yagi for 40 meter on a 100' tower so I know what to expect of a beam. This thing will not even come remotely close.
But like I said, if would play just as well a rotatable dipole, I would be satisfied.

You mentioned that changing the length of the reflector might help? Did you perhaps mean "radiator" instead of "reflector"? Changing the length of the reflector makes no sense to me, but then again, I now very little about antenna systems. Wink

A hexbeam does fit a full size radiator and reflector for 20 meter so in theory it should also fit a full size radiator for 40 meter. The radiator would have a very odd shape though but I wonder if it would work and if so, would it be better then the shortened radiator/reflector design?
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PE5T
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« Reply #3 on: March 23, 2013, 03:46:12 AM »

Maarten,

In my opinion it is not worth the effort. This beam is a big compromis, difficult to tune (elements are very close), has a small bandwith, brings you only a few dB gain (when tuned properly!) and some F/B over a limited bandwith (to me, the 11 dB F/B mentionned in the article, however not demontrated in the NEC diagram, seems very optimistic). A director instead of a reflector may be a better choise. Besides these considerations, if you are limited to a hight of 11 meter it is still is a NVIS antenna and it could also frustrate the performance of this Hexbeam on the other bands.
I am sure it will not outperform a good vertical or even your end-fed.

Kees
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G3TXQ
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« Reply #4 on: March 23, 2013, 03:57:44 AM »

I did some modelling and experimental work on small, inductively-loaded, hexbeams here:
http://www.karinya.net/g3txq/hexbeam/small_hexbeams/

The 40m design you quote is "three times compromised": the elements are bent, as on any hexbeam; they are inductively loaded; the Reflector is further folded in towards the centre-post which changes the normal hexbeam tip-coupling arrangement. I came to the conclusion it was a step too far!

I would want to confirm the authors comment that there is no interaction with the other bands if I was going to implement it.

Steve G3TXQ

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PD2R
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« Reply #5 on: March 23, 2013, 05:06:55 AM »

Kees and Steve,

Thanks for both of your helpful reply's. it's clear to me now that this isn't worth the effort. I was kind of expecting this before I asked the question. A 7(!) band Hexbeam sounds to good to be true.
There is and old saying that is applicable to this matter: If it sounds to good to be true, it usually is!  Wink

Thanks again, I will stick with my Endfed wire for the time being. Maybe I will put up a free standing vertical in the backyard one of these day's. Putting down enough radial will be a challenge but at least I know it will work.

73, Maarten PD2R
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W5DXP
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« Reply #6 on: March 23, 2013, 09:41:42 AM »

I actually wouldn't mind if it would do just as well as a rotatable dipole.

I didn't mean to imply that it would work as well as a rotatable dipole, just that it would beat the fixed dipole in the dipole's worst directions. It will considerably worse than the fixed dipole in the dipole's best directions.

Quote
You mentioned that changing the length of the reflector might help?

It's just a thought and I have not modeled it. Seems to me that the terrible F/B ratio might be caused because the reflector is not the correct electrical length.

Quote
A hexbeam does fit a full size radiator and reflector for 20 meter so in theory it should also fit a full size radiator for 40 meter.

That idea didn't occur to me. If you used the entire hexbeam support for a single-wire dipole, it might work. Perhaps someone will model such an antenna. Of course, at a 11m height, it wouldn't make a lot of sense.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
PD2R
Member

Posts: 131




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« Reply #7 on: March 23, 2013, 10:19:37 AM »

Hi Cecil,

I get your point. My fixed endfed works pretty well in most directions which probably has to do with the fact that it is so low to the ground.
The only advantage of having the dipole folded in de Hexbeam is that it would clean up my back yard which would make my wife happy. Wink
To some that my not be a good reason, to others it might be the only reason.
I think I can get away with one wire in the back yard so I'll stick with my end fed wire  Grin

Thanks for help.
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