To: | [email protected] |
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Subject: | Re: LF: TX > RX isolation test |
From: | N1BUG <[email protected]> |
Date: | Wed, 18 Jul 2018 17:34:51 -0400 |
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Hi Stefan, > Be happy to find some new challenges to expanse your knowledge spectrum. I am, but I feel like a fool asking so many questions and sometimes having difficulty understanding how things work at this part of the electromagnetic spectrum. > Here i have the same experience with Linux, a big struggle to me! :-) I understand! I use linux every day but for routine things like email, web browsing, etc. For this I do not need (and do not have) much 'under the hood' expertise with linux. >> In hindsight, the TX> RX antenna isolation measurements I made the >> other day make no sense at all to me. I now question whether the >> results mean anything. >> > Your measurements appeared meaningful to me. Perhaps so but now I do not understand how the coupling can be only -21 dB. The efficiency of the TX antenna is < 0.1% and surely the little RX antenna is very inefficient also. How can I have only 21 dB loss of signal between them? What is the method of coupling or signal transfer for antennas in such close proximity? > But as far as i understand your main interest is to reach the best > sensitivity of the RX antenna when it is standing far away from the TX > antenna. Yes, that's correct. >> If the preamp is designed for 50 ohm input, why is a 50 ohm resistor >> not a suitable 'dummy load' for receiver testing? >> > It is. But do you think that the antenna and the 100:1 transformer > represents a Z=50+j0 Ohm load? Ah. I see. In fact I have no idea what complex impedance it presents. > All in all i would favour the JFET preamp solution but if you like, you > can also treat the RX antenna like a TX antenna and resonante it (using > a coil on a ferrite rod for example) to 137 kHz and then transform to 50 > Ohm and connect your 50 Ohm preamp. Your transformer will have a winding > ratio of just 4:1 maybe, e.g. 20:5 turns. I would like to continue using this antenna on LF and MF, at least until I have more RX antennas installed. The preamp I use now is (perhaps foolishly) band pass filter, preamp, and two way splitter all in one box. This has been good because with the filter I can TX will full power on 160m, HF, VHF without interference to my LF and MF receivers. If I understand correctly, one suggestion is to convert this antenna into an e-probe by eliminating the transformer and putting a JFET preamp at the antenna. It would be reasonably simple (maybe) to feed DC power over coax. I don't have any RG58 but I have a lot of RG-59 and RG-6 direct burial cable. I use it for my 160m RX antennas. One concern for me is that I don't make my RX situation any worse than it was last winter. To address that, maybe I can leave the existing RX antenna just as it is and make a new one with JFET amplifier and power over coax feed at the more distant location where this one failed to work. I don't know if that is a good idea. Until now I did not see anyone use long coax runs with e-probe type antennas. Loss is obviously not a concern, but common mode problems could be. The coax to reach that location would be about 100m. I see most people put e-probe antennas very close to their home with short coax runs. I can try that and see what happens. I also read the suggestions to find a quiet location for the antenna. My experience at 160m is probably useless at LF, but I have always found that any RX antenna in close proximity to my house (and neighbor's house) was a disaster on 160m. Very noisy! > Yes yes it is a good idea to use a directional antenna. > But, a normal loop will also improve the situation and is easier to > build. the K9AY stuff is more complex/ advanced. But certainly a good > choice. A loop is also in my plans. I have many plans, not enough time or money. :-) 73, Paul N1BUG |
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