Re: polarization

[Bernard Cleyet <anngeorg@PACBELL.NET>]

Yeah,  folded dipole is 300 Ohms; a 1/4 w/ ground plane is, if I
remember correctly, 55 or 75.

[That's why 58 or is it RG 59/U (and RG 8 & 9) has that impedance, and
twin lead is designed to be 300-- this reminds me of a book set named
lost technology.  I just received one -- it's what I did in the 50's.
God I'm getting old!]

I already answered this problem on the original thread: use a dipole
receiver to check (detmn.) the polarization.  Since the output of a 723
is ~ 25 milliW.  one must use a diode and EMF detector (DMM) instead of
a lamp.  I haven't tried it yet, but I suspect a bare 723 w/ a gnd.
plane might work (the gnd. plane produces an image, so it acts "like" a
dipole.

There exists wave guide to coax adapters.  3 cm is long enuff so one can
make a balun to drive a dipole (folded or not).   Consult any old? Radio
Amateurs Handbh.  The Cenco:

http://www.mjs-electronics.se/images/Diverse/80422.jpg

Is another soln.  Possibly available on the used market.  I understand
it's S-band ~ 10 cm. [thanks to a PHYS-L member.]

BTW, Pasco makes a microwave dipole (they claim the diode is the same
one they use in transmitter.  (maybe they mean the wave guide recvr.?)
I found it rather ineffective, until I cut away some of the traces to
make it shorter *.

Another example of Pasco's nodding:  They claim their IR detector is
"good" to 20 microns. w/ a sapphire window -- I pointed out they mean 2
microns.  The newer ('03) catalog is to 30 microns;  KBr or salt????
The IR thermometer Radio Shack has on sale this month is rather good.
Unfortunately it doesn't have a separate preamp.  Just one chip to:
reference, preamp, and drive the display.  I hope soon to remove the
sealed gas filled thermopile -- At $40 it may be the least expensive
source for one.

* The war surplus 723 stuff isn't interchangeable w/ Pasco's.  I had to
use a receiver w/ moveable short in order to tune it to receive the Gun
xmitter.

bc, who again announces: has complete [except power supplies] x-band
demo. sets;  inexpensive (very) or trade for your surplus stuff.

p.s. Leigh describes another Cenco app.  In that case,  the power (801A
tubes in push-pull) is sufficient to power a flash (torch for LP) lamp
in the middle of the dipole (not folded) receiver at, at least, 5
meters.  The wave length, however, is, like JC's, ~ one to three meters.



Leigh Palmer wrote:

> Brian Whatcott suggests:
>
>
>
> > If waveguides were too rarified, perhaps a twin line with dipole
> > termination would be more to the student taste... Its polarization
> > is less easily gainsaid, possibly?
>
> Yes, that would be good, though I don't know what you mean by "dipole
> termination". I wouldn't like to have to manipulate the "cake grids" of
> size sufficient to do the demonstration with the long waves I envision
> coming from a simple half-wave dipole. The grid must be large enough to
> transmit a significant beam power and also block signal from
> diffracting around it. X-band microwaves are an ideal collimated source
> for domestic cake grids in this respect.
>
> Can someone suggest an appropriate dipole radiation source and
> receiver? John Cochran, a colleague of mine here at SFU, once made a
> Hertzian dipole oscillator transmitter and a receiver based on a folded
> dipole about 1.5 meters long. The receiver had a diode and a (battery
> biased) neon lamp at the feed point. The receiver was mounted so it
> could be rotated from vertical to horizontal, demonstrating the
> polarized nature of the radiation from the Hertzian oscillator. The
> demo worked well, but no cake grids were (nor could have been)
> employed. I don't know why he used a folded dipole instead of the
> conceptually simpler simple dipole. I asked him and he doesn't
> remember, either. The voltage developed at the feed point should be
> twice as high with a folded dipole, I think.
>
> Leigh