Dish feed selection philosophy for EME Stations

Introduction

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 feed you should go for, is a choice that probably is unique for your situation and what you want to achieve.

It almost becomes a philosophic question.


Goal

What is your goal? Get the best possible system performance, of cause. But, in what way? You might need to do some compromises along the way.

As written before, one extreme would be to strive for maximum antenna gain what so ever. One example of this is the W2IMU feed in a 0.6 f/D dish. This was an excellent choice and probably the best choice at the time when this feed was designed. You could probably get a noise figure of your 23 cm LNA of 1.5 dB or if you had a parametric amplifier you might achieve close to 1 dB NF. So what you wanted was maximum gain from your antenna. But this feed in a 0.6 f/D will not be optimized for lowest possible noise. Today, with LNA noise figures getting close to 0.1 dB you need (or have the possibility to) to think in another way.

Do you want to optimize for getting heard by other stations as good as possible?

Do you want to hear your own echoes the strongest possible?

Do you have ample of Tx power and want to be able to pick up the most faint signals from small stations?

Do you want to have the lowest possible antenna noise performance?

But, bear in mind, the differences between some of these cases are quite small but still worth considering.

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 W2IMU feed in a 0.5 f/D dish and 0.5 dB NF will give optimum receive performance (according to OE9PMJ calculation, 1984). If you have lower NF a lower f/D will give the optimum Rx performance.

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 feed

In 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!

Links

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 site

This 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