eHam
eHam Forums => Satellites => Topic started by: 5B4AHO on March 02, 2023, 04:48:36 AM
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Hi All
I am just getting into Sat operation and wish to work with a small rotator in my garden (SARCTRAC).
I have an FT847 and will be using an Arrow dual band antenna.
I am a little confused as to how to connect a setup like this.
As the FT847 has two separate antenna connections for 2m and 70cms, would it be better to run two separate coax cables direct to the radio from the Arrow without any duplexer (would this cause desensing on full duplex?), or would it be better to use two duplexers, one at antenna end to get down to one coax and a second at radio end to split single coax to the two separate antenna connections.
Any suggestions would be appreciated
Cheers
Baz, 5B4AHO
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Many years ago I tested the isolation of the two antennas. I don't remember the figure but I do recall it's pretty good - the elements are 90 degrees from each other and there's quite a bit of frequency separation. From there it would depend on the out of band rejection of the radio, which may not be a published number. Using two diplexers would certainly work but will add cost and a bit of loss which maybe you don't care about, but I would try it without and see how it performs with terrestrial signals and see how it does.
Mark K5LXP
Albuquerque, NM
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Thanks Mark for your comments, am currently trying it with two duplexers that I had in the shack, seems to work ok, may try it with a short run of two coax see how it goes. Tried and got a contact through the ISS crossband repeater, but was not hearing my self coming back in the full duplex, I think I was on about 20w (in error!) so will try with lower power say 5w as I believe higher power increases desense.
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My experienced suggestion is TWO feedlines AND put a duplexer on EACH feedline (only using one side of the duplexer) at the antenna end as filters. This way, you can add a preamp at the antenna later, if needed. (radio > coax > (preamp) > duplexer > antenna)
The reason for this is 1) the duplexers will help filter out the transmitter signal, both broadband noise AND 2m third harmonic when on V/u birds (the 2m uplink third harmonic falls very close to many uhf passband downlinks, SO-50 for one), 2) if placed between a preamp & antenna, the preamp will not amplify such noise (as opposed to having the filtering back at the radio) and 3) separate feedlines will help prevent the transmitter to receive interference and allow the use & powering of preamps through the feedline.
That said, many use a single Elk or Arrow to work satellites. If it is handheld and the coax is only a few feet, not a problem. Even in those cases, whether I use an Elk or an Arrow with a built in low power duplexer (I have two of each), I still have a duplexer just before my rigs to help with filtering. (I also find manipulating a single coax so much easier when handheld.) BUT, when you move the antenna out into your garden, the much longer coax is going to introduce loss on UHF downlinks. This is where a UHF preamp is a huge advantage. And you also need to consider, that unlike with handheld operations, the polarity of the antenna will be fixed. This will, at certain times during the pass, introduce a serious loss of signal. It is not an end of the world issue, BUT be prepared for the periodic nulls as the satellites rotate. And again, a UHF preamp will help with this as it will make the nulls seem shorter.
If you have any questions, please ask. I miss home operations as I have moved and the new QTH is not satellite friendly (high horizons, lots of trees). I am hoping to be more active as a portable op again in the near future. Some of my previous ops if interested: https://www.qrz.com/db/N4UFO/P (https://www.qrz.com/db/N4UFO/P)
Oh, by the way, I helped get an Elk antenna donated to a neighboring school to use with a SARCTRAC rotor system that they built as part of a robotics class. For a while SARCTRAC seemed to be shut down due to parts shortages... glad to see them back in production. If only I had the spare funds. :) EDIT: They seem to be currently sold out. :(
P.S. I used to have an FT-847 for satellite operations. If using the original hand mic on SSB, I recommend upgrading to a condensor mic (Technofix makes a nice one). Also, the FT-847 has an internal preamp you might want to turn on ("INT"). OR the 847 can provide 12V up the feedline for a masthead preamp, but I powered mine separately and had both turned on.
73 & GL! Kevin N4UFO
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Hi Kevin
Thanks for info, will try as you suggest also will check your website. Only got my SARCTRAC last week, came fully assembled, works well in the garden with SATPC32 and I will be trying portable as well.
Regards
Baz
5b4aho
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Two coax lines should work fine. I used a FT847 with a Cushcraft A270-10S where both elements are in the same plane, and ran separate coax lines, and could hear myself just fine on the downlink on satellites when on full power. The ISS does present a bit of a challenge as that frequency combination does seem to lead to desense.
73 John AF5CC