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Author Topic: Small HF yagi, is there such a thing?  (Read 2407 times)
VE3PLO
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Posts: 158




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« on: July 28, 2009, 11:02:31 PM »

i own a 20m dipole and a hustler 4btv with FEW radials, hustler is mounted on a fence 3 feet away from garage at about 5 feet from the ground, the antenna is grounded and as i said has few radials due to space limitations.
I have been listening to my radio for about two months now, roughly about 2 hours everyday and the only station out of north america i heard was in northern Ireland. I have to say i am not satisfied with my my 4btv setup(lack of radials, mounting location etc..) nor with my dipole.

I would like to have a beam for some DX work, but am unable to errect a tower and get a full sized one. I saw something called a portable yagi made by super antennas, model yp-3. Would this work for DX? 6 feet of off the roof on a tripod?

Is there something else out there similar?
Any recommendations?
Thanks,
new to Ham Radio guy;)
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KB6VIV
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Posts: 195




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« Reply #1 on: July 28, 2009, 11:08:53 PM »

The size of Yagi's is pretty much fixed relative to wavelength.  But google hexbeam and moxon if you want to broaden you scope a bit.
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PD2R
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Posts: 131




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« Reply #2 on: July 29, 2009, 02:37:53 AM »

I think your best bet would be a Hexbeam. Small, lightweight, no traps, low wind load and a good performer.
I have not yet build a Hexbeam but I have build 2 Spiderbeams. Both are wire yagis but the Spiderbeam is much lager. It's one of the best, if not the best lightweight yagis out there.

I have a TH2 MK3 from Hy-Gain which does a reasonable job. I choose the Hy-Gain because I think the Hexbeam is quiet ugly. We do not have any trees surrounding our house so everything that sticks out from the roof is highly visible.
If the Hy-Gain is to be replaced, I would now go for a Optibeam OB6-3M.

Please keep in mind that any antenna which has a shorter radiator the half a wave length always will go at the expense of the performance.
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WW5AA
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« Reply #3 on: July 29, 2009, 06:36:12 AM »

Your BTV is mounted wrong it seems. If the base is 5' off the ground the few radials you have are useless even if elevated. The only way to mount a vertical is a few inches above ground or at 15'/20' min. above ground with raised tuned radials. In my opinion you would be better off getting the BTV installed properly. Have fun!

73 de Lindy
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KB6VIV
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Posts: 195




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« Reply #4 on: July 29, 2009, 07:05:08 AM »

Would you classify a hexbeam as a Yagi?
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WB5JEO
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Posts: 805




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« Reply #5 on: July 29, 2009, 07:23:31 AM »

A 4BTV will do a perfectly adequate job, when it's given a chance. The two options are:

1. Elevated it enough that your radials (typically two for each band) aren't detuned by the earth. How high depends on whether you intend to run the radials horizontally or droop them to bring the feedpoint impedance closer to 50 ohms.

2. Ground mount with as many wires installed as a ground screen as you can manage. 32 is a good number, 32 feet or so long, folded if you don't have that much space in one or two directions.

But I've used the 4BTV is all sorts of configurations that did NOT give it the best chance, and it still performed, including DX. For example, ten feet up with random length radials incorporated in the mast guys. And ground mounted without radials, adjusted as per the maker's recommendation for that situation. The worst of these was the no-radial ground mount, which is to be expected, but at least as important was that it was in a congested area, surrounded by three buildings and some trees. But it still worked.

I think your main 4BTV problem is that the low mounting places it in an unworkable position. The radials can't produce a ground plane, because of earth coupling. And if you ran a ground wire to earth level and put the radials there, it's even worse.

Space limitations are difficult, but not insurmountable. While it's best to arrange radial symetrically around the base, when it can't be done, you can use tuned radials arranged as best you can. Same for ground mounts. Put in what you can. For instance, if you can get it well above the garage roof, you can use non-conductive guys to the other side of the garage and arrange tuned radials where you can (ends not too close to a metal fence). You can try conductive radials over the garage roof, but they will interact with a metal roof, but you may be able to play with their lengths to compensate. It's really a forgiving antenna, but there are just some configurations that work hard against it.

All Yagi's and similar antennas are sensitive to height above ground, and Yagi's shortened by traps and such have the same height requirements as full sized. What you suggest may actually work better than your 4BTV as it's mounted, but that's a very poor comparison when the 4BTV is crippled by its installation. You can spend considerable money on a Yagi and think you've done good, but it may only be in comparison to what is right now essentially a random vertical stick. And, of course, your Yagi won't get on to the lower frequency bands.

I suspect your dipole also suffers from low height. Height affects take-off angle, and for intercontinental DX, you need it to be fairly low, which means greater height - or a vertical. Read up on the Web and The Antenna Book on what makes 1/4L verticals work. In constricted space, you have to make some compromises, but if you understand what you're trying to accomplish, you can usually get something that works.
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N3JBH
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« Reply #6 on: July 29, 2009, 07:59:44 AM »

I have my 6BVT mounted 3 inches above the ground and it works very well for DX. I also have a very small Cushcraft MA5B yagi that does fairly well for a compact yagi. Other antennas in use at this QTH cant be discussed on here hi hi.  Jeff N3JBH
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PD2R
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Posts: 131




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« Reply #7 on: July 29, 2009, 08:54:49 AM »

KB6VIV:
ya·gi   (yä'gē, yāg'ē)  
n.   pl. ya·gis
A directional radio and television antenna consisting of a horizontal conductor with several insulated dipoles parallel to and in the plane of the conductor.

The conductor is more or less horizontal in a W shape and it has parasitic element (reflector) to provide directivity.

http://www.qsl.net/wy3a/G3TXQ_Broadband_Hex.htm

So yes, in my book that makes it a yagi ;-)

73, Maarten
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N3OX
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« Reply #8 on: July 29, 2009, 08:55:47 AM »

"Would you classify a hexbeam as a Yagi? "

I probably would.  I'd put it in a "generalized yagi" class where you start with a parasitic beam with linear elements and bend the elements for various desirable properties; size reduction and to tweak the coupling.  

The Moxon the same way too.

"I would like to have a beam for some DX work, but am unable to errect a tower and get a full sized one. "

Forget about a beam for the moment.  You should be able to work the world from VE3 with a dipole or well installed vertical and 100W.

While it is technically possible to build a decent beam with tip to tip length of 1/4 wavelength instead of something closer to 1/2 wavelength, with three elements on a boom that's 0.14 wavelengths long, it's hard.  

I think the YP3 gain over a dipole on 20m is zero.  It says "6" presumambly 6dB over "something" in the manual, but they do not tell you the something, and looking at the gain curves as we go up in frequency it suggests that we are looking at the gain in dBi INCLUDING ground reflection gain with the antenna mounted at a fixed height.

If that's the case, the YP3 has 0dBd gain when mounted at the same height as a 20m dipole.

Plus, it's really a single band at a time yagi.  You can't use it on all the listed bands with quick changes.  You have to take it down and measure and adjust and tune it for each band you want to use.  Not very good for a home station antenna.

A beam is a very nice thing to have, but I don't think that your basic problem will be solved by throwing money at the problem, and I think you have to be careful with short/small antennas, because there's a cloud of gimmick and incomplete info surrounding them.  

I think you should work on getting some good DX heard and worked with your existing antennas.     I worked 200 DXCC countries from Erie, Pennsylvania starting at the absolute bottom of the last sunspot cycle with little more than a big random wire, a 20m dipole, and a 17m ground plane antenna.

You need to make sure your existing antennas are set up well, and tuned right.  If you're going to use the vertical to transmit it does need to have a reasonable set of radials, either tuned for the bands and elevated or a large number of whatever length you can fit in at the base of the antenna.   The antennas you have when set up right will be plenty effective for DXing.

And you also need to get on the right times of day.  Do you get on the same time every day?  The bands come and go to DX locations.  If you get on at, say, lunchtime every day, the stations on 20m will probably be pretty weak and tough to work.  When are you getting on?

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WX7G
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Posts: 5908




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« Reply #9 on: July 29, 2009, 09:54:23 AM »

I wonder if you have something more going on than a modest antenna. You should be able to receive many, many stations on 14 MHz all day long. 40 meters day and late into the night. And 80 meters at night.

Even an inferior antenna will allow you to hear hundreds if not thousands of stations every day. A lossy antenna will still give a sufficient signal-to-noise ratio.

So, could it be your receiver? The coax? To find out plug in a 15' length of wire to the receiver antenna jack. You should hear plenty of stations on 40 and 20 meters. If not, try another receiver.
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KB1LKR
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Posts: 1899




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« Reply #10 on: July 29, 2009, 10:03:07 AM »

Would you classify a hexbeam as a Yagi?

If you were rigorous in your classification scheme no; but for practical purposes it's essentially a 2 element (driven element & reflector) Yagi w/ its elements bent in a few places. RF doesn't care about classifications.

There was an article in the last annual antenna issue of QST describing a broad banded hex multi-band beam but you could just build the 20m section I suppose. It's a little larger (21' across vs. 19') in trade for more BW & better F/B.  

Same for a Moxon, like a 2 element Yagi but w/ bent elements. W1DYJ had a 20m Moxon construction article in QST last winter too.
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WB5JEO
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« Reply #11 on: July 29, 2009, 05:19:40 PM »

"ya·gi (yä'gē, yāg'ē)
n. pl. ya·gis
A directional radio and television antenna consisting of a horizontal conductor with several insulated dipoles parallel to and in the plane of the conductor."

The Yagi-Uda patent depicts reflectors arranged in a vee, like a corner reflector. So, it looks like it's dipoles coupled by fields that makes it a Yagi.

Poor Uda-san. It pays to have your name listed first.

Or maybe you just have to have a more or less exotic name to have an antenna named after you. Yagi, Marconi, Beverage, Sterba.  Brown's antenna just became known as the folded dipole. Moore's became the quad. Of course, there's also Bruce. I guess it was too hard to come up with another name for it.
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VE3FMC
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« Reply #12 on: July 30, 2009, 06:20:52 PM »

I have a 2 ele triband in use. It was originally a 3 ele TA33JR but I dropped the director as it was damaged.

Mine is currently only up 21 feet and it works but not as good as it would at 50 + ft.

If you are going to mount a yagi on a tripod on the roof why not mount your 4BTV on the roof and run 1/4 wave radials for each band on the roof.

It would work just fine. You lack of radials and having it mounted on that fence are likely the main issue.

I can not tell where you are due to your user name, but I have to assume you are in Ontario? Contact me and maybe I can help.
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WB6BYU
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Posts: 12985




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« Reply #13 on: July 30, 2009, 07:38:26 PM »

You can make a small HF yagi, but performance drops off
rather quickly as you shrink it.  The late W4RNL did a lot
of analysis on small yagis, which you can find on his
web site at www.cebik.com/radio.html (you have to register,
but it is worth for the wealth of information available.)

As a quick summary, you can't shrink the boom length much
and still maintain performance.  You can shorten the
elements to about half the normal length before the losses
overcome any potential gain.  (In some circumstances there
may be an advantage in having a directional pattern even
if the maximum radiation is less than a dipole.)


A dipole should hear plenty of DX if it is reasonably
high and properly installed.  You may be listening at
a time of day when the bands aren't as open, or on the
wrong part of the band (DX stations tend to cluster near
the bottom, often below the US phone band.)  But DX stations
will also tend to be weaker, and more easily buried under
the mass of US stations, so you may have to listen
carefully to hear them.
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