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Author Topic: Some Questions About Short Boom 6M Yagis  (Read 7037 times)
K0OD
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Posts: 2591




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« on: May 11, 2010, 01:27:36 PM »

Been getting my feet wet on 6 meter SSB/CW DXing. Have worked about 8 states and Mexico this past week using 100 Watts and a rotary dipole at 30'. I'm now looking to buy/build something better, with an eye on the June VHF contest.

Desired antenna will be hand rotated and limited to 20' to 35' up with a 5' to 9' boom. I'm only interested in working the bottom portion of the band.

1) How important is above-ground height on 6M for sporadic E? Note that a heavier antenna will have to be closer to the ground.

2) With a 2-4 element Yagi, should I design for maximum forward gain or for best back and side rejection (mainly to reduce receiving noise)?

--
I'm thinking of buying the tiny (2.5 ft boom) Diamond 2 element HB9CV and adding a single additional  director. Such a hybrid would have a boom just over 6' long and should have a spectacular f/b of about 25 dB, with forward gain around 6 dBd.

Ideas?

 
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WB2WIK
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« Reply #1 on: May 11, 2010, 02:10:17 PM »

You might try a 6m Moxon, which is smaller than a Yagi and works quite well.  A project description is here:

http://www.n2mh.net./moxon.htm

Height always matters, just get it as high as you can!
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WB6BYU
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« Reply #2 on: May 11, 2010, 02:19:55 PM »

Quote from: K0OD

2) With a 2-4 element Yagi, should I design for maximum forward gain or for best back and side rejection (mainly to reduce receiving noise)?

Gain is probably more important on 6m, while G/T is a better measure on the higher bands.  But don't
get so fanatical about gain that you end up with too low of a feedpoint impedance, which makes
matching more difficult and increases losses in element resistances.

Quote
I'm thinking of buying the tiny (2.5 ft boom) Diamond 2 element HB9CV and adding a single additional  director. Such a hybrid would have a boom just over 6' long and should have a spectacular f/b of about 25 dB, with forward gain around 6 dBd.

It might, or it might not.  Adding parasitic elements to a HB9CV changes the phasing considerably,
and it is difficult to predict the result on the antenna without modeling it.  W4RNL developed a
similar antenna by adding a parasitic director and got good performance:

http://www.cebik.com/content/a10/phase/phagi.html

But when the driven element was removed it turned out that the HB9CV was actually tuned for
maximum gain rather than F/B - the director was providing the F/B.  In any case you'd want to
do some serious modeling of the antenna to confirm that the commercial HB9CV had the proper
characteristics to start with.

However, you can get close to 6dBd from a number of standard 3-element yagi designs without
having to build the phasing harness.  For example, check out DK7ZB's short-boom 3-element
6m antenna designs:

http://www.mydarc.de/dk7zb/start1.htm

G0KSC has a number of different designs here:

http://www.g0ksc.co.uk/

Designs from either of these sites are capable of equivalent (or better) performance compared to
the hybrid HB9CV + director. 
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K0OD
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« Reply #3 on: May 11, 2010, 04:14:08 PM »

Thanks for the tip on the impressive 3 el LFA antenna at:
http://www.g0ksc.co.uk/3el-19mtr-boom-lfa.html

Yes, it's pretty easy to get 6 dBd gain but so many 3 element yagis fall way short in f/b. On the VE7BQH yagi comparison table, the popular 3 el Cushcraft is only rated at 9 dB f/b.

Why are almost all those f/b readings low, including the G0kSC 3 element yagi? KSC says his design has 30 dB f/b but VE7BQH puts that number at just 13.6 dB  (or am I reading the table wrong?)

http://www.vhfdx.info/VE7BQH6.html
 
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WB6BYU
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« Reply #4 on: May 11, 2010, 04:57:01 PM »

I'd have to go back and check, but I think it is the difference between front to BACK and front
to REAR ratio.  The latter looks at the overall radiation in the rear hemisphere rather than just
straight off the back.  You can adjust a yagi for a very deep rear null and have a good F/B,
but the radiation elsewhere can be high enough to have a poor F/R (for example, if there are
two strong rear lobes with a sharp null between them.)

Optimizing yagis for gain generally brings up a lobe right off the back of the antenna, making the
F/B lower.  But if that is the only rear radiation then the F/R can actually be pretty good.
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K3WACKY
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« Reply #5 on: May 11, 2010, 05:42:58 PM »

It's a little late, but a 2 element quad is my preferred choice for a short boom directional antenna on 6M.

It could be harder to make depending on your skill level, but you may want to look into it in the future.

I agree the Moxon is another great choice. I made one of those too and it works great.

73 and good luck in June
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K0OD
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« Reply #6 on: May 11, 2010, 07:58:04 PM »

I'd have to go back and check, but I think it is the difference between front to BACK and front to REAR ratio.  The latter looks at the overall radiation in the rear hemisphere rather than just straight off the back. 
Hey Dale you're right!

That table does say "F/R" not "F/B."  Eyes aren't great, but I admit I wasn't familiar with the term, F/R. But I'm quite aware of the limitations of the simplistic pre-eznec F/B ratio   

Also says in the table's footnotes:
"F/R is Front to Rear in dB over the rear 180 degrees of an antenna using either E or H plane. "

--
Moxon and quads. Considered both. I thought I could come up with a better performing antenna than a Moxon in a slightly larger package. And  wasn't sure who made a VHF quad kit nowadays. I wanted it shipped immediately.


 
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N8EKT
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« Reply #7 on: May 12, 2010, 09:04:13 AM »

MFJ sells a 3 element 6 meter beam with a 6 foot boom for $79.95
And Blue Star antennas sells a well made 4 element for $120
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WB6BYU
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« Reply #8 on: May 12, 2010, 09:42:23 AM »

3 elements on a 6' boom can give you 8+dBi gain and good F/B (or F/R). By varying
the length of the director you can optimize it for F/B or gain.  Impedance varies
between 20 and 12.5 ohms in the process.  For your application, the higher gain
12.5 ohm might be the best choice, which is easily matched with a pair of 50 ohm
coax cables in parallel.
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K0OD
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« Reply #9 on: May 13, 2010, 08:12:44 AM »

I probably shouldn't have dismissed the Moxon.

Spent a few hours last night modeling various short boom beams and the 2-element Moxon offers a lot: it beats much bigger antennas in the important f/r department. Under some QRN/QRM conditions they should hear better than some much larger yagis.

Moxons are also cheap, light (3 lbs on six), low profile, broad-banded and can be made multiband and multi-element if desired. I always considered them to be just mini-2 element yagis but they appear to have excellent qualities of their own.  In some ways they model better than regular 2-element yagis, which I find amazing.

I vaguely recall Moxon-like antennas in ham literature years ago but they were offered as inferior versions of conventional 2-element Yagis for those with restrictions.  I'm guessing that the Moxon has come into its own because the antenna really shines in computer modeling.
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N3OX
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« Reply #10 on: May 13, 2010, 09:27:14 PM »

Moxons are also cheap, light (3 lbs on six), low profile, broad-banded and can be made multiband and multi-element if desired.

Don't forget to lay eyes on a model for that!   I think some of the multi-band and extra-element Moxon designs that are available (for example, on  the Moxon Antenna Project page) haven't really been well tested and modeled compared to the basic two element W4RNL-optimized version.  Do the "multiband and extra element" guys show the plots?

Quote
I'm guessing that the Moxon has come into its own because the antenna really shines in computer modeling.

The close coupling afforded by the tails is not just a space saver.  It's important to the good operation of the antenna.   I think the "Moxon Renaissance" has a few factors, and the excellent F/R is one of them, but don't get too lured in by that either. The excellent F/R makes it seem very beam-y.   And man, it saved my 6m operation a couple times in an apartment.  But honestly, front-to-rear isn't even the most important thing for reception!  it really depends on how your noise is distributed.

If your noise is uniformly distributed from all azimuth and elevation angles and your antenna is highly efficient, then the best measure of your reception improvement is the antenna gain.  It's not really the gain, per se, but the reduction in signal in all directions except the one you want to listen in.  If the noise is coming from "everywhere", any rejection from any direction is equally useful, whether it's reduction of signal 30 degrees off boresight or 180 degrees or 120 degrees in a rear quadrant.

For poor efficiency antennas, it's important to normalize this by the overall efficiency so you can compare.  The "Receiving Directivity Factor" that you can find information about on W8JI's website takes this into account.  But if you're comparing antennas that all have exactly the same efficiency, the RDF and the gain at the desired azimuth/elevation will all compare the same.  And if you look at the RDF of various low band arrays, you can see that excellent front-to-rear stops mattering so much once you have, I dunno, 10 or 15dB worth of it.  At that point, your reception starts to be limited NOT by stuff off the back, but by the undesired stuff off the front.

Ultimately, if your interfering signals and noise are coming from everywhere, a higher gain antenna is going to hear better because the pattern is tighter.  You don't need more gain in principle for better reception, but at VHF it's easy enough to tighten the pattern while keeping efficiency high.  It's really only if you have localized noise off the back when you're pointing in a desirable direction that high F/R matters.  That's probably likely enough in suburbia to make a super F/R antenna worthwhile.  

But it's only in that situation, noise off the back, that they would hear better than larger yagis.  When you can just barely hear that JA at 330 degrees over the neighbor's  TV at 310 degrees, you'll wish you had a bigger beam :-)

-----

The mechanical advantages are pretty good.  They're cheap and light and small turning radius... all nice.  

But one of the biggest advantages, IMO, is that you can toss one together that works right the first time with whatever junk you have lying around, because the Moxgen program exists.  You don't have to mess with guessing how to scale to new wire or tubing size... you can just punch the numbers in and build the thing.  If you build it to within a few mm, it'll just work.    Here's the measured pattern of my 15m Moxon.  I built it on the shack floor, strung it up.  Done:

http://n3ox.net/files/15_moxon_measurement.jpg

(I'm going to post that again for ya in a different thread  Grin )

I think the smallish turning radius, the fact that they have their own design program, plus the fact that good F/R makes people feel very successful in their beam construction is a lot of what's driving the popularity of the antennas.  I have four of them :-)  

But, in the end, they are similar in a lot of ways to a two element yagi, with a broad forward beam and not a whole ton of gain.      So if you can, you might want to look at a bigger antenna.

I built a Moxon-spreader-esque five element wire yagi for 6m a couple seasons ago, something like a 15 foot "boom" :

http://n3ox.net/files/sixyagi_lg.jpg

It was so Moxon-esque that now the spreaders are holding that 15m Moxon instead.  The 6m yagi actually worked pretty well but due to mounting constraints, I was only getting about 4dB better at low angles (would have been 6, had they been at equal heights) and I also had to beam out over the house while not quite clearing it with the 5 element antenna... and it was too close to the siding, and all kinds of stuff.   All in all I decided it wasn't enough better for my purposes to keep it instead of building a better antenna for 15.

I'll probably try the principle again sometime.  I think it could make a pretty good portable collapsible big yagi if I did some tweaking. It's extremely lightweight.  It's not perfect: it's really easy to break the crappie pole spreaders I used, and I need to work out the details of how the connections to the support ropes might be affecting the apparent element lengths... and I want to model the sag of the first two directors to do some sensitivity analysis.

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

Monkey/silicon cyborg, beeping at rocks since 1995.
N2SLO
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Posts: 66




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« Reply #11 on: May 14, 2010, 05:06:29 PM »

It's a little late, but a 2 element quad is my preferred choice for a short boom directional antenna on 6M.

It could be harder to make depending on your skill level, but you may want to look into it in the future.

I agree the Moxon is another great choice. I made one of those too and it works great.

73 and good luck in June


 :)I agree! I have a Cubex 2 Element Quad up at 25 feet. VUCC #1592..... I talk to everybody I can hear. I love my QUAD, great signal to noise ratio, quiet, and I have gain... I also have an 8 element Quad for 2M SSB. Go to www.cubex.com.. good luck.. David  
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N6ORB
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Posts: 244




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« Reply #12 on: May 21, 2010, 02:43:43 PM »

It's interesting that the Moxon option came up. As you know, we worked each other last night, May 20, and I was using a Moxon at 13 feet (HOA antenna restrictions.) It was a bit of a struggle, but we did complete the QSO, San Francisco area to St. Louis. The Moxon I use is made by Par Electronics.

I'm now at 274 grids confirmed over the past four years using this portable setup. While the Moxon works well for portable and rover operations, I'd still want a Yagi with a rotor if I didn't have the antenna restrictions.

Dave
N6ORB
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K0OD
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« Reply #13 on: May 24, 2010, 10:00:48 PM »

Thanks for the contact Dave.  San Francisco is about my best DX but I've worked about 8 Qs with California.  I bought the two element Diamond a502hb and am up to about 23 states now. Propagation has been excellent most days.

I ran two tests to determine the Diamond's f/b. Transmitting thru the yagi to a field strength meter about 100 feet away showed a f/b of about 21 dB 

I also used an MFJ-259 SWR analyzer to generate a signal that was transmitted via a resonant dipole to the Diamond 100 feet away and my Flex-5000 with its very accurate dBm-calibrated S-meter. That test showed an astounding  f/b of 29 dB and f/side of about 24 dB.  I repeated that test and got about the same result. Certainly impressive for a yagi with a 2 1/2 foot boom.

I'll probably eventually toss up a reference 6 meter dipole and perhaps a home made moxon  for gain comparisons.

----

The Diamond is going to remain a 2-element Yagi for the time being. I modeled it with one additional director and a single dB of additional gain came with a significant hit to f/b. Adding a reflector seemed to be the slightly better way to go.
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KB1GMX
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« Reply #14 on: May 28, 2010, 08:13:07 PM »

In order of preference from least to better.   Best is provisional..

 Dipole
 2 element director/driven yagi.  typically 3ft boom. 
 Moxon (best small antenna, narrow and short)  Good for tight spaces maybe portable ops.
 3 element yagi (6ft boom around 7-8db)  generally a good starter beam,
 4 element yagi short 115inches (arrow 52-4)
 4 element long yagi 14-16ft boom
 5 element yagi  18-22ft boom.

   Quads for the most part are ok but eat vertical space and stand out visually. 
   Weather can eat them, here in New England we have ice and big wind and
   quads don't like that.

  Longer the boom the greater the gain and more elements to make a good pattern.
  All including the dipole need a rotator.
 
However with all of those also a halo or turnstile for omni coverage and listening for other directions.
Stack two halos or turnstiles for some gain.

No matter what you do higher is better but in clear flat areas 20ft will do.

I currently run a short boom (homebrew) 4 element and plan a modified CC A505 with a longer boom and
element locations (w5VWO mod).   

When 6 is open any wet noodle is fine and will work europe.  When it's barely open then gain and pattern
counts big time

 
Allison
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