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
Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
One stop wish list for any gift,
from anywhere, for any occasion!
http://thewishzone.com