Re: 3.4G feed horn dimensions ?
Neil Smith G4DBN
A simple round horn has a wide pattern and large side/back lobes, so it doesn't illuminate a shallow dish very effectively. That means the horn is probably seeing hot ground, as well as lowering the efficiency of the dish/feed system.
Fitting a choke ring or rings suppresses the sidelobes and helps
narrow the pattern somewhat.
The section of guide ahead of the probe helps to ensure that higher modes are suppressed and that the wavefronts coming out of the tube are pretty much planar so you will get an in-phase reflection across the whole of the dish. Rule of thumb is at least half a waveguide wavelength between the probe and the open end, but you can model it in OpenEMS or something to be sure.
The VE4MA style chokes are much simpler to make (and smaller)
than a Chaparral, and you can adjust the width, depth and offset
from the open end of the tube to fine-tune the pattern and
sidelobe suppression. W1GHZ's site has a lot of analysis of how
to match the choke topology to your dish f/d. On a prime-focus
dish, you have to take into account the extra blockage caused by
the choke. http://www.w1ghz.org/antbook/chap6-3.pdf and there are
good articles in DUBUS.
Without a choke ring, if your dish has an f/d of less than 0.4, you won't lose much on transmit, but you might have a little extra noise on receive. If you are using an offset with f/.d 0.6-0.8 then it is probably worth fitting a choke. It won't make any noticeable difference to the match.
A VE4MA choke ring looks like this, although I usually TIG-weld the rings to the horn - this one needed to be removable for fitting:
Fitting a scalar choke like this one (similar to the Chaparral in
concept, providing a high-impedance surface of 1/4-wave (ish)
slots) does clean up the pattern very well, but the blockage is
too high for a small PF dish under 1.5 m or a 1.2 m offset
On 06/12/2020 14:34, Nick Gregory G0HIK via groups.io wrote:
The 9cm's SG-Labs arrived yesterday and I'm cant wait to get going.
-- Neil http://g4dbn.uk