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Author Topic: cob web antenna for qrp?  (Read 3130 times)
W1JKA
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Posts: 1662




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« on: December 11, 2012, 11:38:26 AM »

    I would appreciate any comments from cob web users here in the states.There quite popular with my 2x qrp dx contacts but I have not had any contacts with cob web users here.I am considering homebrewing a single element one for 30 meters.Thanks  Jim
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WA2TPU
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Posts: 209




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« Reply #1 on: December 11, 2012, 12:35:55 PM »

To W1JKA.....Jim I had one up for 10 thru 20 meters a couple years ago. It worked fine on those bands...its very light weight...easily turned with a tv type rotor and an ok Dx antenna. I took it down and replaced it with vertical and horizontal polarized Delta loops that are fixed in various directions. Delta loops, vertical arrays and Sterba Curtains are my favorite antennas tho.
I think a 30 meter Cob-web antenna has some interesting possibilities IF you can get it up in air high enough.  Good luck with your project.
Best regards and many 72/73.
Don sr. --WA2TPU --
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WB6BYU
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« Reply #2 on: December 11, 2012, 05:13:17 PM »

The cob web antenna is basically a dipole bent into a square.  Gain will be
slightly down from a dipole, but generally not enough to worry about.
They have the advantage of being light and reasonably small.

In many cases I'd go with an inverted vee off the same mast instead, as it
is simpler.  But if you don't have convenient spots to tie off the ends of
the elements or otherwise need the antenna to be self-contained and rotatable,
it isn't a bad option.
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W1JKA
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« Reply #3 on: December 12, 2012, 03:14:52 AM »

    Don and Dale:Thanks for your timely replies.As I understand it,cob webs are somewhat omni directional compared to a longwire or dipole type antennas and am interested in knowing if there is any noticeable difference in noise level between the two,your particular Qth and ground conditions withstanding.
     Regarding my post,a little background info:Due to limited clear space and support considerations I am basically limited to horizontal wire or or relatively small diameter beam type antennas.I only operate qrp on 20/30/40 meters with a dedicated rig for each.Over the course of three years I have built and done A/B comps with about every type wire antenna out there,all of them started out at a height of 38-40 ft. Bottom line is I ended up with a 66 ft. long new Carolina windom that is most efficient for me at a 25 ft. height for dx purposes,this is fine for 20/40 meters but I do not like using a tuner which I have to in order to use 30 meters on this type of windom.
      I also constructed a single element 20 meter hex beam which is as effecient as my windom with the added benifits of being a bit quieter and rotational ability,again I had to lower this from 40 FT. down to about 23 ft. for best efficiency.It would be impractical for me to take down the hex and reconfigure it with an additional 30 meter element thus my interest in the cob web in preference to a dedicated tunerless 30 meter wire type which due to proximity may cause unwanted interaction with my windom.
                                                            Jim
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WA2TPU
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Posts: 209




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« Reply #4 on: December 12, 2012, 11:26:17 AM »

To W1JKA....

Jim I wonder IF you have enough room to put up a 30 meter Half Square???The reason I ask is I've found them to be a pretty good antenna either at Qrp or Qro levels. I last week  worked the ZL9HR Campbell Island DXpedition on 30 meters using a Half square and 5 watts. And NO!! I'm not bragging....I'm just sharing with you and others on this forum what antennas I use to get my DX.

A Merry Christmas...festive Holidays to you and yours Jim.
Best regards and many 72/73.
Don sr. --WA2TPU --
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WB3BEL
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Posts: 81




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« Reply #5 on: December 12, 2012, 11:39:38 AM »

 ...I also constructed a single element 20 meter hex beam which is as effecient as my windom with the added benifits of being a bit quieter and rotational ability,again I had to lower this from 40 FT. down to about 23 ft. for best efficiency.

If you are saying that your 20m hexbeam works better at 23ft than 40ft for long distance QSOs then you are mistaken.

I see lots of posts talking about antenna "efficiency".  There are lots of efficient antennas that are not good antennas.  The engineering definition of efficiency is that the antenna radiates a strong electromagnetic field relative to the applied source.  That radiation may go in a direction you don't care about like into the clouds or down into the dirt.

...It would be impractical for me to take down the hex and reconfigure it with an additional 30 meter element thus my interest in the cob web in preference to a dedicated tunerless 30 meter wire type which due to proximity may cause unwanted interaction with my windom.                                                   
Jim

Depending on what supports your hexbeam, you could use it as a vertical radiator against a ground system.

I am not sure why you don't like using a tuner, but if it is fear of introducing significant loss, then it probably is unfounded if you have a half decent tuner and reasonable load impedance at 10 MHz.  If it is because it is a nuisance to tune and you only use it on one band then it seems like you could leave it set and bypass it on the other bands where its not needed.  But if you have to change it multiple times for different bands, I agree it can be a bit tedious.  Maybe I am missing your point?
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WA2TPU
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Posts: 209




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« Reply #6 on: December 12, 2012, 12:33:01 PM »

to WB3BEL....

That was a darned good post you made....very interesting point of view and truthfully stated.
Yep! There's a lot of efficient antennas out there in engineering terms but I think it still comes down to your location and band conditions.

Best regards and many 72/73.
Don sr. --WA2TPU --
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WB6BYU
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Posts: 13243




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« Reply #7 on: December 12, 2012, 01:04:49 PM »

Quote from: W1JKA

...most efficient for me at a 25 ft. height for dx purposes...


...again I had to lower this from 40 FT. down to about 23 ft. for best efficiency...



I also would question the use of the word "efficient" here.  Do you perhaps mean "effective"?
How are you measuring efficiency?

With horizontal polarization the angle of radiation drops as the antenna height is increased
because of the interaction with the ground reflection.  Increasing the height from 25' to
40' often makes a big improvement in low angle radiation for working DX, though there may
be specific stations or paths that rely on a higher angle that don't work as well.

If this really is the case, then I'd suggest that you have significant common mode current
on your coax that is responsible for the radiation.  In that case, the vertical radiation pattern
from the coax acting as an antenna would raise the angle of radiation as the coax is made
longer.  A good friend used to work DX on 40m with a dipole in the rafters of his garage, about
12' off the ground.  One day he plugged in a 20m dipole to an unused port on his antenna switch
and the SWR went haywire.  Turns out that high common mode currents on the outside of the
coax were responsible for most of his radiation, and he had changed the "antenna" when he
connected the shield of the new feedline.  Things calmed down after he added a balun to
the antenna, but he could no longer work any DX, because the antenna is too low without
the help of the vertically polarized radiation from the feedline.

Certainly what is now called a "Windom" (an Off-Center Fed Dipole:  the original "Windom"
had a single wire feeder) is renowned for common mode currents, even in spite of the addition
of a choke to try to reduce them.

That's not to say that, in your specific situation, there might not be some reason why you get
better results at a lower level due to local buildings, terrain, power lines, etc.  But that certainly
isn't the experience of most hams, so one tends to look for other explanations. (And, of course,
it depends on what you consider to be "DX".)



Quote
As I understand it,cob webs are somewhat omni directional compared to a longwire or dipole type antennas..

Slightly more omnidirectional and slightly lower gain than a dipole.  A little more loss, too, because
the radiation resistance is much lower - about 12 ohms, requiring a step-up transformer of some sort.

I wouldn't expect any difference in noise other than that due to being able to null out
noise sources off the ends of a dipole, or otherwise due to differences in the pattern.
If you have common mode current, you may be picking up noise on the outside of the
coax as it passes by all the electronics in the house, in which case the antenna with
the best common mode rejection (improved by adding an effective balun) probably will
have the lowest noise.

Over the years I have noticed that antennas fed against earth ground at my location
tend to have a higher noise level than those that are balanced (even a dipole fed
without a balun.)  This appears to be due to a high level of electrical noise coming in
on the power ground lead, possibly from an arcing connection that I can hear on a
clear night from 1/4 mile away.  But when the antenna is isolated from ground, the
noise level is often S1 or S2 with most of the antennas that I've used.


If you are only using the Windom on 40m and 30m, then a trap dipole may give you the
best performance, or two dipoles (one for each band) on a common feedpoint.  Or you
may be able to add 30m wires in parallel with the 40m wires on your current antenna.
If vertical polarization works better in your situation then a half square or "T" antenna of
some sort might worth trying.

It may also be practical to string a 30m dipole around the outside of the 20m Hex frame,
somewhat like a Cob Web but a bit larger.  I'd have to do some modeling to see how
much impact that has on the 20m pattern.

I used to operate mostly on 40m, and had good results even with antennas at relatively
low heights.  I learned a lot when I started using 30m, because the antennas that worked
well for me on 40m were no longer effective due to the high angles of radiation.  (It gets
reflected back to Earth on 40m, but not on 30m.)  I ended up putting up a homebrew
vertical, though getting the 30m antenna up to 40' would have helped as well.
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W1JKA
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Posts: 1662




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« Reply #8 on: December 13, 2012, 07:13:48 AM »

    As always gentlemen your responses are appreciated,interesting and informative.Re:Don,I did make a 20m halfsquare that evolved into a 20m inverted U (Not V) dipole for sole MM use on my small sailboat.WB3BEL is also correct as to my misuse of the word "efficiency" I should have simply said "MY best results". I am not surprised that my "lower is better" results raised an eyebrow or two,it surprised me also because at my alternate qth,portable,ect. the standard mantra of higher is better mostly holds true.
    My definition of qrp dx is outside CONUS,and I am aware that my Carolina windom is not a true windom,perhaps I should have used the word "variation"  in lieu of "type." Nothing against tuners here,I have built several and have an old Drake MN-7.Since all my antennas are cut to my frequencies of interest I have yet to find the advantage in useing and/or carrying them around for the majority of my operating needs,(exception and reason as per my previous post).
    Besides messing about with antennas and on air operating my other favorite part of qrp ops is my continuing study of propagation.I have three years of detailed logs pertaining to conditions at time of each contact.Propagation and chasing the grey line both dictate as to when and what bands I will be operating.This with a little luck makes my qrp dx niche all the more interesting.Meanwhile back to the ant farm and my
30m antenna project,I have gleaned from all your insight and suggestions that I will also be considering other options for same.Thanks again for your thoughtful replies and happy holidays to you all.       Jim
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