[Moon] [Moon-Net] 1296 EME

[Ingolf, SM6FHZ]

Dave and all.

For a small dish at 1296 MHz (~<2.5 to 3m) a conventional feed horn can be
somewhat bulky and cause some blockage, this feed might be an effective
alternative:

http://www.2ingandlin.se/Circularly%20polarized%20patch%20feed%20for%201296%20MHz_A.pdf

G4BAO, John and SM6PGP, Hannes are both successfully using it with ~2m
dishes. They use slightly different schemes to generate circular
polarization. Hannes is using the scheme shown in the link whereas John is
using a branch line coupler (3 dB hybrid) directly as described in RadCom
in October and November 2015. Both ways have pros and cons.

It should also be noted that a small dish (<10 to 15 lambda) with a large
f/D (~>0.4) is difficult to feed in the low noise way as Paul describes.
This is due to the feed size needed to get a narrow enough beam to get a
quiet dish system. Avoid small dishes with f/D >0.4 if you would like to
get a low noise receive system.

There is, as Paul says, a lot to gain from carefully choosing a feed that
suites your dish and ambitions.

73 / Ingolf, SM6FHZ

2015-12-20 3:56 GMT+01:00 Paul Wade <w1ghz.vt at gmail.com>:

> you've all convinced Dave that a dish is good,
> but let's tell him why:
>
> at 1296 MHz, and other microwave bands up to 10 GHz,
> sky noise is very low, under 10K.  so what you can hear
> is really limited by your receiver noise figure, and by
> ground noise picked up by your antenna.  a dish does a
> better job of keeping ground noise out of the feedhorn
> than the sidelobes of most yagi antennas.
>
> with a quiet antenna and a good preamp, when you do the
> numbers, each tenth of a dB of NF makes your G/T
> (antenna gain divided by receiver noise temperature)
> worse by roughly a dB.  since any combiner and harness
> must be in front of the preamp, the loss adds directly
> to NF, while a dish can have the preamp right at the feed
> with minimal loss.
>
> the result is that for EME, you will hear better with a dish,
> even with the same gain.  and more gain is easy - a bigger
> dish.
>
> at 1296, nothing else is that critical.  careful
> engineering and attention to detail will give good results.
>
> 73
> paul
>
> On Sat, Dec 19, 2015 at 7:15 AM, Dave Sublette <k4to at arrl.net> wrote:
>
>> Good morning,
>>
>> I have “dabbled” in EME, off and on, for several years.  I have made a
>> few QSOs on the horizon with my terrestrial weak signal gear.  I have had
>> successful QSOs on 144, 432 and “maybe” on 50 and 1296 Mhz.
>>
>> Lately I have been considering putting a modest installation up for
>> 1296.  I’ve been reading several articles I’ve found through web search.  I
>> get the impression that to work 1296 one needs a minimum of a 10 or 12 ft
>> dish using circular polarization.  There appears to be a definite bias in
>> favor of dish over multiple yagi antennas.   I had initially hoped that I
>> could put up four, 55 element, loop yagi’s and point them with a modest
>> az/el system without totally breaking the bank.   One article I read left
>> me the impression that multiple yagis was a waste of time and money because
>> you can’t get the theoretical addition al 3 dB gain for doubling the number
>> of yagis.  Plus, the losses are extremely difficult to avoid within the
>> harness and power divider system.  Perhaps I misread.  I’m not trying to
>> generate any controversy, but I have to ask before investing in a system.
>>
>> So what do you think?  How many stations on 1296 are using horizontal
>> polarization?  How many are using multiple yagis vs installations using
>> dishes?
>>
>> On the dish side of the issue is the type of feed.  It seems the pitfall
>> here is getting the antenna optimized.  The construction and placement of
>> the feed appears to cal for sub-millimeter tolerances or efficiency and
>> match, noise figure and all these vital elements of a good system rapidly
>> deteriorate to the point where, once again, one has wasted time, effort and
>> money to obtain mediocre or disastrously poor results.  It’s almost enough
>> to dissuade me from starting the project. I suspect I’m not the first to
>> feel this way.
>>
>> It makes me wonder how all these QSOs are being accomplished.
>> Obviously,  many have “ignored the facts” and gone ahead and put up
>> “something”, with success.
>>
>> I’m not without experience and test equipment.  I run multiple yagis on
>> VHF and UHF. I have operating systems on all bands through 3 CM.  I have
>> built all my yagis, power dividers, and harnesses on 50-432 Mhz. I have a
>> complete test bench.  But, I would like to hear from you who have the
>> experience in 1296 EME.
>>
>> Thanks & 73,
>>
>> Dave, K4TO
>> EM77wx
>> Kentucky
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-- 
Ingolf, SM6FHZ
http://www.2ingandlin.se/SM6FHZ.htm