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Re: Re: look this dual band egg



   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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