Setting the right QRG on EME

The following discussion about how to set the right frequency on EME took place in the Moon-Net reflector. I found the subject so interesting that I decided to put together all the messages in this page.

On 2 February 2000 VK2KU  wrote:
I have a question about precise sched frequencies.

Suppose I have a sched on 144.120MHz and I am to transmit first. Suppose further that it is moonrise here in VK, and the Doppler shift for my own echos is +400Hz; for my sched partner Fred in the USA, viewing the moon higher in the sky, his shift is -200Hz (to make the numbers easy). Between us the Doppler shift is 200-100 = 100Hz (+/-).

Then if I set my transmit frequency to exactly 144.120000, then I will (maybe!) hear my echos on 144.120400, and Fred will hear me on 144.120100.

Fred will then transmit on 144.120300 so as to hear his own echos on 144.120100 (on top of me), and I will hear him on 144.120400 on top of my own echos.

So Fred should listen for me originally on 144.120100, somewhat higher than the sched frequency, and he needs to know the relative Doppler shift (or else my Doppler shift as well as his own) to calculate this.

Is this what happens in practice? Ok, many of us can't measure the frequency that accurately (but many can), and the effect is not very great on 144MHz (but more so on 432MHz). So the question is more one of principle.

The only frequency Fred and I have in common is 144.120200, which is the frequency which a listener on the moon would measure for both of us. I transmit 200Hz below this, and listen 200Hz above it. Fred listens 100Hz below it, and transmits 100Hz above it.

It would obviously be simpler if the sched frequency of 144.120MHz was the frequency for a listener ON THE MOON. Then I know to transmit 200Hz low (144.119800) and listen 200Hz high (144.120200). Fred knows to listen for me 100Hz low (144.119900)  and transmit 100Hz high (144.120100). Maybe this is in fact what happens. Is it?

Would the experts please enlighten me. None of the EME primers I have read seem to address this. I can imagine that at 10GHz it would be quite important to get it right!

On 3 February 2000 W1JR  wrote:
My opinion to this question is that I always transmit on the exact frequency of the schedule (like your example of 144.120 as best I can set my tx) and assume that the other station does the same. Then, all we have to each do is to offset our receivers to compensate for doppler. To do any more just complicates the already complicated situation. You already know the approximate the doppler shift. Furthermore, depending on the Moon's position, it may be varying lots so why complicate it further?

Just my opinion.

On 3 February 2000 G3SEK  wrote:
On skeds, station A (the first to transmit) always transmits exactly on the named sked frequency.

If station B doesn't hear A in the first period, he also transmits exactly on the named sked frequency. Both stations then stay on the same frequencies, and hope to find each other.

If B does hear A in the first period, he has two choices. The aim is to give A the best possible chance to find him.

1. B's TX should have already been tuned to the named sked frequency, before the sked started. If A appears pretty much where you expected him to be, this is the best bet.

2. B can retune his TX to land his incoming signal on the same frequency as A's own echos (see below). This may be a better bet if you find A a long way off frequency.

It's easy to land your signal on the same frequency as the other station's own echoes. You don't need to know anything about the Doppler shift.

1. Offset your RX clarifier to receive your own echoes at your favoured audio pitch.

2. Tune the band with the MAIN tuning (don't touch the clarifier).

3. When you find a signal, tune it in to your favoured pitch using the MAIN tuning.

4. Hit the key - your signal will arrive at the same frequency as the other station's own echoes.

To prepare for a sked where you're station B (the other station transmits first) you should already have the RX clarifier set up to hear your own echoes with VFO A. If there aren't any echoes, offset the clarifier according to the computer output.

VFO B is set to transmit exactly on the named sked frequency, using an external counter directly on the output signal - DO NOT rely on the transceiver dial readout!

When the sked starts, tune for station A using the MAIN tuning (don't touch the clarifier). At the end of the first period, you then have a simple one-button choice.

If you didn't hear station A, press the SPLIT button and transmit on the named frequency - and stay in SPLIT mode for the whole sked.

If you did hear station A, you can either press SPLIT or go straight ahead and reply on top of his own echoes. It's a matter of judgement and experience to know which will give the best chances of a QSO.

On 3 February 2000 OZ5IQ  wrote:
As G3SEK Wrote , I do as follows:

SKED FREQ.

1. Find your echos with the RIT tune the MAIN VFO to the echoes is on the sched freq.

Random:

 As 1)   and then tune the MAIN VFO to the wanted freq.
 
AFTER THE QSO has begun - do NEVER touch the main vfo agn.

On 3 February 2000 DK9ZY  wrote:
Right,

Joe I fully agree with you.That's my practice too.

It can be recommended for EME stations that they should determine their precise frequency by using a good (calibrated) frequency counter. Praxis shows frequency offsets of up t0 500 Hz,excluding Doppler.

It's not enough to use a TCXO on the IF (short wave tranceiver),the most critical frequency uncernity,in my case, is the transverter.My LT2-S takes approx. 50 min to warm up and runs within that time 500-700Hz. If I have to start a EME sked without warm up time,I really need my external counter to stay on (TX) frequency.

BTW: I personally prefer to use the RIT for the receiver NOT the main tuning,because the range of the main tuning is too big and you always have think about your center frequency. The RIT range can be set to +- 1.25 kHz.This gives (in my opinion) very comfortable tuning.

On 3 February 2000 VE7BQH  wrote:
Before this frequency discussion gets to far off base, I at least want to clearly state how frequency is set on 2 Meters.

1. SCHEDS:

Frequency is set at the transmitter to the pre chosen frequency, ie 144.119 MHz NO adjustment is made for doppler at the transmit end.

Adjustment for doppler is made at the RECEIVE end only.

2. RANDOM:

On random, set your frequency by zero beating your echoes to the station you are about to call if you are lucky enough to be able to do this. If not set your freq to allow for the doppler shift.

What you do if you find a station well off frequency becomes a catch 22! Do you move? Do you stay put? My experience has been to stay put. I have lost more contacts by moving than staying put. Make your best guess! The good thing today is it is very rare to find a station well off frequency so it has become a non event.

On 3 February 2000 SM5BSZ  wrote:
Lionel wrote:

>2. RANDOM:
>
>On random, set your frequency by zero beating your echoes to the station you are about to call if you are lucky enough to be able to do this. If not set your freq to allow for the doppler shift.

Very important, add a random shift within +/- 150Hz,(maybe a little more, but be sure to never be as much as 500Hz) to the frequency Lionel describes.

Two stations on the same frequency within 20Hz was the most common reason for unsucessful random contacts, more common than insufficient signal levels during the last years with my 4x14 antenna. (In contests - that is when I called CQ a lot)

On 3 February 2000 AL7EB  wrote:
Thanks, Lionel,

I recently was thinking about this subject and wondering what the standard was.  During the second ARRL weekend, when I was briefly QRV-QRP, I now realise I operated completey wrong.  I didn't consider Doppler at all and just tuned in the station I wanted to call.  How dumb.  But I'll guess most newbies may do the same.  Fotunately the experienced operators I called found me, or guessed I was dumb and what I was doing [or I was lucky and the Doppler wasn't too much].  FFTDSP really shines here!

I will print off this info and put it in my 2m folder.


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