EME using small antennas

 

For me it was always very interesting to learn what minimum equipment is needed to make EME QSOs, especially on 23cm. I extended a W2IMU feed made of 6mm mesh by adding a large cone, achieving a horn antenna with 80cm mouth diameter. Estimated gain is 19dBi, maybe slightly less.

A spare 80W SSPA was used for the transmit side. Together with the horn I got 49dBm+19dB=68dBm EIRP.

On the receive side a cavity preamplifier with a MGF4919 was used.

23cm horn antenna with 80cm diameter

horn80cm

 

listen to the reception of ON0EME

 

First tests were made in combination with my 7.3m offset dish. I was transmitting with the dish, reducing transmit power step by step down to 10W. At this level I still could occasionally see traces of the echos on Spectran waterfall.

Next was receiving the beacon ON0EME with the horn antenna. The signal was found immediately, clearly visible on Spectran and WSJT, audible most of the time when it transmitted carrier. CW was not readable, only sometimes a letter or fragments. The level in WSJT was between -20 and -22dB, on occasion -19dB. Probably I loose 1dB or 2dB due to some extra noise pickup because of the broader lobe.

       
 

Measuring echo strength with different antenna combinations

    

Weak but traces detectable: echos at my 3m dish when transmitting with 80W at the 80cm horn

 

 

Next: transmitting again with 80W into the horn, receiving with 7.3m offset dish

audio in SSB bandwidth: echo horn 80W to 7.3m dish

audio in CW bandwidth: echo horn to dish filtered

 

Finally: transmitting with my 7.3m dish and full power, receiving with the 80cm horn

SSB bandwidth: echo 7.3mdish to 80cm horn

CW bandwidth: 7.3m dish to horn filtered

Testing echos was good to learn what is possible with such a small station, and it was also nice to confirm previously made calculations. But making QSOs with that setup was another challenge.

During the 2016 ARRL EME contest I worked within a couple of hours on Oct 22nd several stations in WSJT using the 80cm horn antenna and 80W, all random:

0646 HB9Q (-9 -21), 0658 UA4HTS (-12 -22),

0709UA3PTW(-13 -19), 0746RA3AUB(-20 -24),

0811 DF3RU (-20 -?), 0831 OK1KIR (-17 -23)

I did not use deep search and deleted call3.txt. HB9Q has a 10m dish, all the other stations I worked have dish size around 6m. As we can see from the receive reports between -12 and -20 for them, it should be possible to decode stations using a 3m dish and good power.

I even copied several big guns in CW.

Two of them I recorded:

CQ de OZ4MM

I1NDP in QSO with IZ1AEM
       

As a result from my measurements, together with experiences from the past, I tried to find a simple formula to calculate what equipment is necessary on 23cm to detect echos, make QSOs in WSJT (assuming decode at -25, no deep search and call3.txt), CW and SSB (depending on personal skills).

Calculation is based on antenna gain of station A(dBi), antenna gain of station B(dBi) and power in dBm. For example, a 3m dish has about 30dB gain, a 6m dish 36dB gain, 250W is 54dBm. The result in this case is 30dB+36dB+54dBm=120dBm.

Necessary values on 23cm are:

echo traces on waterfall: 96dBm

WSJT: 102dBm

CW: 112dBm

SSB: 122dBm

(all values +/-2dB, to stay on the save side add another 3dB)

 

Some examples:

Own echos just visible on waterfall:

7.3m dish, 100mW ...... 38dB+38dB+20dBm=96dBm

3m dish, 4W ...... 30dB+30dB+36dBm=96dBm

3m dish to 80cm horn, 80W ...... 30dB+19dB+49dBm=98dBm

 

WSJT:

HB9Q 10m dish received OE5JFL 7.3m dish, 200mW -25 ...... 41dB+38dB+23dBm=102dBm

several 6m dish stations received OE5JFL 80cm horn, 80W between -24 to -19 ...... 36dB+19dB+49dBm=104dBm (1dB to 6dB margin to the necessary -25!)

 

CW(random!):

19.March 2000: ZS6AXT 5m dish; OE5JFL 1m horn, 1kW ...... 34.5dB+20.5dB+60dBm=115dBm

3.Oct.2010: DL0SHF 9m dish; OE5JFL 2.8m dish, 16W ...... 40dB+29dB+42dBm=111dBm

 

 

 

Long time ago (1999 and 2000) I already had made similar experiments

To demonstrate what is possible with EME, I made moonecho tests on 70cm between my 10m dish with 31dBi, and a dual-dipole antenna with 10dBi, widely used as a feed for a dish. The audio samples below demonstrate the signal quality when transmitting with the big dish and receiving with the small antenna ( and vice versa).
On Nov.28th 1999 I worked DL9KR (O/O) on random within 5 minutes, using the NBS antenna. Maybe this is the smallest antenna ever used to make a 2-way random EME QSO with a non-commercial station.
After this success I wanted to try similar on 23cm. A cone was soldered to a circular W2IMU horn, resulting in 1 meter horn diameter and a gain of about 20.5 dBi.

 

 

 
TX with DISH , RX with HORN
RX with DISH , TX with HORN
HB9BBD received with HORN

On March 19th 2000, I worked on 23cm 10 stations random within 3 hours, transmitting with this small antenna. Highlight was the QSO with ZS6AXT, who is using a 5 meter dish. I could even hear my moonechos receiving with the big dish, when I transmitted with the 1 meter horn and only 50 Watts.
Here you can see the ´big´ horn made of mesh wire. It is 2.3m long. The picture on the right side shows this ´EME´-antenna in its operating position: feedpoint indoors, and the cone looking outside through the shack window.