"Chris W" wrote in message
news:MQN8f.5728$ZP1.1737@dukeread11...
Dale Parfitt wrote:

"Chris W" wrote in message
news:ku88f.1019$ZP1.519@dukeread11...

Dale Parfitt wrote:


PS I would like to build 2 of these one is for 2.4ghz wireless network

and one is 432 mhz amateur band EME and or satellite work. So the
2.4ghz version will likely never see more than 100 mW but could see as
much as 1 watt. The 432 mhz version could see as much as 1000 watts,
but if I did that it would be to an array of 4 of them so I guess each
one would only see 250 watts?

--
Chris W



EME on 432 is linearly polarized (although switching polarities to make
up for Faraday rotation can be advantageous) - you'll be throwing away a
huge 3dB, have higher sidelobes than a well designed Yagi (nullifying
the low sky temp advantage of deep space) and less gain per boom length
than a good Yagi- all in all, a dismal choice for EME.

Dale W4OP



The calculation in the ARRL Antenna book say that a 12 foot boom with a
circumference of 1.33 wavelengths will give me a 19.9 dbi gain this
should make up for the 3 db loss I think. I haven't seen a yagi that
gives that much gain in with a 12 foot boom. The book also says that the
side lobe problem can be significantly reduced by creating an array of 4
helical antennas. That should up the gain to 25.9 db. That's pretty
good for EME isn't it? Take it to 16 and you get over 30 db gain. And
the only need to be spaced at 1.5 wavelengths, that's only 3.4 feet.

--
Chris W


The closest optimized Yagi I found was a K1FO on a 14' boom- 17.9dBi
which compares quite well with your 19.9dBi on a 12' boom once we adjust
for the polarization loss making the helical 16.9dBi linear for the helix
vs. 17.9dB for the K1FO My experience with EME is at 23cM where we use
dishes- but I can tell you that 1dB is to kill for..
The circularity could be an advantage as it would be insensitive to
Faraday. Another advantage is that they are non critical to build- at
least from a gain standpoint. But I think the real issue will be weight
and weight ditribution. Between the helical tubing, non conductive boom
and the reflector screen a helix is going to be a clydesdale compared to a
Yagi.
Another problem might be the necessity to mount the array from behind the
reflector- that's going to be quite a moment arm.

Bottom line is, 432 MHz EME is well explored- I am not aware of the use of
helices for EME there- although, again I am not active on 432 EME. If
they are not being use, there's probably a reason.

Dale W4OP

I guess unless both stations were using a helical antenna, an array of
yagies would be better. Since the helical antennas would have to have
opposite polarization, that would be pretty inconvenient unless both
stations had an array of RH for TX and LH for RX, in which case you would
have to build twice as many antennas.

I did have an idea for mounting them though. I was going to find
something to use as a form to wrap the helix around and then cover it with
fiberglass, making a big fiberglass tube, then remove the mold from the
inside. I think that would make for a pretty strong and lite structure. I
would then make some kind of fiberglass structure to mount the antenna at
it's CG. Of course such an antenna would have a pretty high wind load, so
you would need a way to easily take it down in case of high winds.

--
Chris W

Hi Chris,
You're right on the polarity sense- it gets reversed upon lunar surface
reflection. At 23cM we are using a scalar or septum horns that have 2
feedpoints- CW and CCW- much easier to to do with a horn as it only involves
a 2nd orthogonal probe. A set of 4 capacity stubs arranged longitudinally
along the guide generates the 2 senses.
If you are seriously interested in EME- take alook at 23cM. Gain is easy to
come by in the form of dishes; LNA NF are now in the 0.2dB and power from
the GS series tubes and Mistubishi modules are stable and quite easy. Not
unusual to hear SSB stations ragchewing on activity or contest weekends, no
Faraday issues and lots of big gun activity.

Dale W4OP