[amsat-bb] Arrow and EME?
ka1rrw at yahoo.com
Thu Apr 1 05:36:59 PDT 2010
Arrow Antenna and EME:
It is possible to work Earth Moon Earth with an Arrow Antenna.
Of course, it is. You just need to make a schedule with someone on the other side of the link with enough Antenna Gain. You also need to be running the new digital text messaging mode called JT65B
Link for JT65
In this document I will refer to all antenna gain values based on the number of elements.
Example: A Single half-wave Dipole = 0 dBd
Elements 2 = 3 dBd
Elements 4 = 6 dBd
Elements 8 = 9 dBd
Elements 16 = 12 dBd
The Arrow antenna (Model 146/436-10) has 3 elements on the 2-meter band, so its maximum theoretical "Element Gain" is 4.5 dBd.
With the invention of the JT65 protocols, thousands of Amateur Radio stations running a Single Yagi 2-meter antennas, now have access to the Moon and EME contacts.
To make a contact on Moon bounce, the "Total Antenna Gain" from both stations is added up and will need to be in the approximate range of 25-30 dBd. With this gain and the average transmitter power of (100-400 watts) you will have a 10-20+ percent chance of completing a 2-way EME JT65B link, with another station.
Note: There are many other factures used in EME gain calculations. For simplicity, we will just focus on the antenna Gain.
The amount of Total Antenna Gain required depends on the mode you wish to use. The wider the mode, the more gain that is required. The mode JT65B is a very narrow mode and requires less gain. I am not going to go over all of the details of JT65 in this article, look it up.
Gain required by mode: (All values are approximate)
JT65 28-30 dBd (1-Yagi + 4-Yagi)
CW 30-40 dBd (4-Yagi + 4-Yagi)
SSB 40-50 dBd (8-Yagi + 8-Yagi)
FM 60-70 dBd (16 + 24) Guess
The Mode JT65B requires approximately 30 dB of Total Antenna Gain for an EME contact.
If we assume the average 12-element 2-Meter Yagi has 10.5 dBd (round to 11 for easy math) of "Element Gain", then the more Yagi's you stack, the more gain you will have. In EME lingo, 1x12 means, you have One Yagi, with 12 elements, 4x12 means you have a stack of 4 yagis with 12 elements each (48 elements total) and an approximate gain of 17 dBd.
1-Yagi = 11 dBd
2-Yagi = 14 dBd
4-Yagi = 17 dBd
8-Yagi = 20 dBd
16-Yagi = 23 dBd
32-Yagi = 26 dBd
64-Yagi = 29 dBd (W5UN)
A Single-Yagi station (11 dBd) calling a 4-Yagi (17 dBd) station will have approximately 28 dBd Total Antenna Gain. The 4xYagi stations are very common on JT56B EME.
Let's go back to the Arrow Antenna:
We need 30 dBd of total antenna gain.
The Arrow antenna has 4.5 dBd.
The Arrow also has a Maximum power limitation of 150 watts (10 watts if hand held).
If you are running the maximum 150 watts on your Arrow antenna, you should be able to work stations with 32 to 64 Yagi's. There are not very many 64 Yagi stations out there, however Dave W5UN has been active on JT65 EME recently.
If you want more of a challenge, you can try QRP at 5 watts and your Arrow antenna.
Reducing you power from 150 watts to 5-10 watts, will reduce your performance by 12 db.
To compensate for the reduce power, you will just need to find a station with a bigger antenna.
There is another big gun on EME. Two weeks ago, Arecibo was on EME, running CW, working EME stations on the 440 band. The antenna used at Arecibo is a simple 1,000 foot dish. The actual gain for 2-meters is not known, however I will assume it is more than 64-Yagi's.
So here is your chance. Make a schedule with Arecibo and go for QRP, EME, with a held Arrow antenna, or if you know of any good contacts at Arecibo, send me the data and I'll try to arrange a schedule and try it from my station.
A good Receiver Preamp (similar to ARR)
A good SSB 2-meter Transceiver (the best ever made Yaesu FT-736R)
Good Coax (RG-8 coax is 11 Millimeter coax. That is ok for 50' EME runs, for longer runs use 12+ Millimeter coax)
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