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Re: Re: look this dual band egg



Hello John,
I apologize for not responding in a more punctual manner,but I have been 
traveling around this past week and access to the web has been spotty at best.

I wanted to add a couple of comments to your posting about the dual-band 
applications of eggbeaters and their derivations.  I, unfortunately, do not 
have any of my files with me, but I do have an "evaluation" copy of NEC4 on 
my old "emergency" laptop that I just got in my hands this weekend.  Here are 
my comments in brief:

Eggbeater II:  the dimensions given on my web page are about 2 percent larger 
than what NEC4 also predicts (so your scaling is appropriate), but I 
intentionally build the antennas a little large first and then "trim" for 
operating frequency.  I've also found that when I tune one for 145 mHz in the 
garage, it resonates about 146.5 on the roof (10 m elev).  I note your scaled 
Egg II plots agree well with the ones on my web page, particularly the 
free-space elevation (although I cut off the bottom half for clarity).  
Similar to your results, I also found the 70 cm use of the 2 m antenna to be 
a significant mismatch, but am at a loss to explain it.  I suspect since the 
1 wl loops are not symetrical (rectangles in lieu of squares) at four 90 
degree sections, maybe the assumption it would resonate at 70 cm is 
erroneous.  I found (via the model) the 2 m version resonant at 420 mHz with 
an almost 3:1 SWR!  I replicated your "classic" eggbeater without radials and 
confirmed your findings that design will make a useful 3rd harmonic antenna, 
even if slightly high in freq. 

TPM II:  While your free-space Zenith/Elevation pattern appears similar, I 
note your gain figure is considerably less.  I confirmed about 6.41 on this 
version of NEC4.  Also note the antenna is actually mounted in a plane 90 
degrees from your model, but that doesn't matter for this analysis.  When fed 
at 436 mHz, I noted a very nice 2.98 dB  gain pattern, similar to a 2-el 
beam.  This agrees closely with my 
observations while working AO-27.  What does not agree is the the 
simulation's feedpoint impedance of over 150 Ohms--again a 3:1 SWR.  That 
result makes me very suspicious of the software model since I tested this 
very antenna on the air and received a very wide bandwidth of under 1.5:1 
SWR.  As noted previoulsy, I used it for AO-27 contacts on several occasions, 
and you know there is no margin in that sat's downlink for a high SWR.  I 
then added a 70 cm reflector as I suggested earlier, only to find the pattern 
and the match both significantly degraded.  This surprised me also.

I'm not sure where all this leaves us.  I have not been following the BB, so 
if other discussions have evolved, maybe you could share that with me.  
I'm curious if you or anyone else tried the nested eggbeaters you suggested?
73,
Jerry, UA3T/K5OE

 >   I have done some casual testing of exactly the scenario you proposed: 
 > using
 >     a 2m Texas Potato Masher II, which is really an eggbeater variant/
 > derivation
 >     designed to add directional gain to the fundamental eggbeater design 
(see
 >     either the TPM II at http://members.aol.com/k5oejerry/tpm2.htm or the 
Egg 
 > II
 >     at http://members.aol.com/k5oejerry/eggbeater2.htm) on both 2m and 70 
cm. 
 > 
 >     Please note there was not a second antenna built inside the first, as I
 >     used the 2 m driven element as a common feedpoint.  I found the antenna
 >     was, as expected for an odd-wavelength design, resonant at 70 cm also.
 >     What I noted, however, was the pattern was not nearly as directional as 
 > the
 >     TPM II, but did have some F/B (I did not attempt to measure it, but 
would
 >     estimate it over 3 dB--about 1/3 of the TPM II at it's design 
frequency).
 >     Specifically, my setup was a dual-band mobile rig (FT-5100) operating
 >     AO-27.
 >                   [K5OE (re-justified for readability)]
 >  
 >  I've had a special interest in dual-band antennas, and indeed, use a
 >  homebrew dual-band antenna for my satellite work (50 direct feed, no
 >  diplexer).  Any such antenna is almost necessarily a compromise.  I
 >  don't have time to built alot these days, and i'm not inclined to put
 >  together an antenna that i haven't simulated yet (J-poles excepted, as
 >  i haven't figured out how to simulate them well yet).  So i looked at
 >  the conventional eggbeater in free space (or at least, i made an 
 >  attempt to simulate it with NEC2):
 >  
 >    http://www.qsl.net/kd6pag/nec/eggtest1f.html
 >  
 >  Indeed, a 2m eggbeater can be adjusted to resonate decently in the 70cm
 >  satellite, but it's basically a 'sky burner' and is no longer an omni-
 >  when operated at its third harmonic.  Like a 2m ground-plane tuned for
 >  70cm operation (the J-pole form of which is often touted as a dual-band
 >  antenna), it can have decent SWR (and thus won't make one's transmitter 
 >  unhappy) but it's not a very good antenna for terrestrial use at 70cm.
 >  (By the way, a 2m quarter-wave 'spike' does make an interesting omni for 
 >  satellite work for passes above about 15 degree elevation, as one might 
 >  observe from its plots [http://www.qsl.net/kd6pag/nec/2mModGndP.html].)  
 >  Alas, the 'dual-band' conventional eggbeater isn't useful for anything 
 >  that i can think of.
 >  
 >  I looked at the 'Eggbeater II', which under NEC2 (thanks, Jerry, for
 >  having the simulation parameters on-line[!]), didn't look that omni- 
 >  directional to me:
 >  
 >   http://www.qsl.net/kd6pag/nec/eggbeaterII.html
 >  
 >  But i've found that things under NEC2 don't necessarily resonate at the
 >  design frequency (and maybe it looks better under NEC4).  But scaling a
 >  litte bit often helps for NEC2:
 >  
 >   http://www.qsl.net/kd6pag/nec/eggbeaterIIa.html
 >  
 >  That got me the azimuth pattern i was expecting, but it did not seem to
 >  resonate in the 70cm band (or the impedence was sufficiently unstable
 >  that i didn't recognize a resonance).
 >  
 >   http://www.qsl.net/kd6pag/nec/tpmII.html
 >  
 >  ...is the Texas Potato Masher II, as described under the URL mentioned
 >  above.  One scaling improves the azimuth pattern, and has more stable
 >  impedence at 70cm, albeit on the high end in this version:
 >  
 >   http://www.qsl.net/kd6pag/nec/tpmIIa.html
 >  
 >  A compromise in terms of SWR for the two bands gives:
 >  
 >   http://www.qsl.net/kd6pag/nec/tpmIIc.html
 >  
 >  Again, while the SWR may be plausible for both bands, the pattern at
 >  70cm, especial in terms of azimuth, does not seem very useful.  So, 
 >  again, it doesn't really function as an effective dual-band antenna.
 >  
 >  I did look briefly at the quadrifial APT antenna, but i had trouble
 >  simulating it with NEC2:
 >  
 >   http://www.qsl.net/kd6pag/nec/apt1g.html
 >  
 >  The elevation pattern looks good for LEO satellite work, but the model
 >  does not seem very stable numerically.  I could be confused, but i think 
 >  that antenna design is based having two loops at different frequencies 
 >  and having the imaginary portion of the impedances cancel out at the 
 >  design frequency.  I suspect that causes numerical problems because one 
 >  ends up adding two floating point numbers of almost equal but opposite 
 >  signs together, and that results in loss of accuracy and a potential for
 >  numeric instability.  I gave up on that approach on two grounds, (1) that
 >  it was hard to simulate, and (2) i was concerned that for similar reasons,
 >  the dimensions for construction might be much more sensitive to errors
 >  in measurement (and i'm not that great at mechanical details).
 >  
 >  Returning to the concept of a member of eggbeater family, i suppose that
 >  perhaps different reflector might improve the elevation pattern for this
 >  family of antenna at the third harmonic.  But i'm quite skeptical that 
 >  much can be done with the azimuth pattern at the third harmonic.  On the
 >  other hand, i am interested in seeing what nesting two eggbeaters might 
 >  do.
 >              -- KD6PAG  (Networking Old-Timer, Satellite QRPer)
 >  

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