Dish feed selection philosophy for EME Stations
Sometimes you see a question about what feed design is the best one. Is it the W2IMU, the VE4MA, VE4MA Super or OK1DFC or perhaps the RA3AQ?
There is no single correct and true answer to this question!It is like when Alice in Alice in Wonderland asks the rabbit in the road crossing: -Which way shall I choose? The answer from the rabbit is: -It depends on where you want to go!
The best heard option
So you want to optimize for getting heard by other stations as good as possible?
You risk to be called names such as "crocodile" (big mouth and no ears) by other stations on EME, hearing you, but unable to get your attention! This will be even more pronounced if you run more power than most other EME stations.
For achieving this goal you should strive for maximum antenna gain when choosing the feed for your dish. Look at the W1GHZ On Line Antenna Book for the most suitable feed for the f/D of your dish. The VK3UM EME Performance Calculator can be used to test different feeds for your working conditions to see the effect on the performance (gain, echo strength, sun noise, signal received at the station etc.).
If you have marginal Tx power this might be a option to consider.
The best own echoes option
So you want to hear your own echoes the strongest possible with the Tx power and Rx NF you have now?
This is a compromise between best antenna gain (best Tx signal) and optimum receive performance. The optimum receive performance is a compromise between antenna gain and low noise pick-up. This is true for a given size of the antenna, if you increase the size of your antenna you will of cause get more antenna gain. Use the VK3UM EME Performance Calculator to test different feeds for your working conditions to see the effect on the echo strength.
You should choose a feed that slightly under illuminates the dish. The optimum degree of under illumination is dependent on the receiver NF you have. See this OE9PMJ calculation for an example.
The best receive option
So you have ample of Tx power and want to be able to pick up the most faint signals from small stations?For this option you should look at a feed that under illuminates the dish. Use the VK3UM EME Performance Calculator to test different feeds for your working conditions to see the effect on the sun noise performance. Look for the best sun noise value.
The lowest noise option
Do you want to have the lowest possible noise performance?
If you go further towards a low f/D dish with a given feed the noise pick-up will go down. And so will the gain of the dish. Note, lowest possible antenna noise is not the same as optimum receive performance. The lowest noise option is probably not of great interest for the EME ham.
Method for choosing a feedIn order to find out the most favorable dish feed for Your EME-system follow the below bullets. They will guide you to a logic method in choice process.
Congratulations, you now have the possibility to build a better EME-system than when you started!
The following links will give more info about antennas and optimizing your EME-system.
W1GHZ on line antenna book.
VK3UM EME Planner and VK3UM EME Performance Calculator can found at SM2CEW web siteThis page is under development and the links may not work as expected. The content of the text will also be updated. Check again soon and they can be updated!
Updated September 14, 2009. http://www.2ingandlin.se/SM6FHZ.htm