RF Feedback
I have recently encountered the dreaded "RF in the TX Audio" syndrome with my Jupiter. So, in the course of seeking answers to that problem, I thought I would share whatever information I discovered here, for the benefit of other Jupiter owners.Feedback through the amp keying jacks
This first bit of information was brought to my attention by Bob, KA1DFO. It was seen on the contesting.com site's ten-tec mailing list, in a post by Mike, W9WIS.Had the same issue with mine and worked for several days with Gary at Ten-Tec who also had one an owner returned on his bench with the same issue. I had redone my ground system, had line isolators all around on every line.... no help.
What was discovered is that there is no RF bypassing on the (TX EN - TX OUT) key lines and rf can get into the radio via that route. It travels directly to the cpu via that route... We tried bypass caps and that didn't help.... then we each soldered in a 100 microhenry RF choke in the amp at the RCA connectors. That helped quite a bit but generally one high freq band still had problems with full output.
The final solution for me was to purchase an AmpKeyer.... and use it via the MOSFET keying relay and opto-isolator chip. That way there is no connection direct to the Jupiter and thus no RF travels past the AmpKeyer..... It even worked when driving my tube amp at 1500 watts...... nothing else worked.
Mike, W9WIS
General RFI control tips
One of the acknowledged experts in the field of RFI, particularly with these radios, is John N0KHQ. John has provided us with the following summary. (I have edited this slightly, for formatting only!)Many thanks to John for this great information!First, the TT SDR's are computers in an RF environment. Special treatment of RF and power requirements apply.
Power requirements:
Your 115vac power source should come from a dedicated single receptacle ie, I use the same 115vac receptacle that our clothes washer is on. It is a dedicated 20amp circuit.
Using a common 115vac circuit will lead to ground loops and noise. There will also be reactive currents on the ground wire due to the load of end devices.
Outdoors, check your service ground wire tied at the grounding rod. This wire should be removed, cleaned and reinstalled and made water tight. A word of caution, if something in your home should short out while your doing this, you'll see the light. Contact a qualified electrician if you feel uncomfortable doing this. If your service ground wire is aluminum, have it replaced with copper wire. Dissimilar metals dont work!
RFI and how to use it to your benefit:
Note: Do not confuse the treatment of RF with the treatment of lightning. They are two different animals.We dont want RF in the shack, we want it outside where it will do some good.
It has been my experience with SDR's that special treatment of RF grounding is called for.
Lines leaving or entering the SDR should be wrapped around a cube toriod such as the kind that RS sells. One per line will work just fine.
From Radio Works, purchase a T-4 Line Isolator. This device is installed right at the rear coax connector of the rig with a short piece of coax and then another piece of coax to your linear or outboard tuner.
What ever you do....do not and I repeat, do not use a common bus bar behind the equipment and then run a wire from each piece of equipment to the bar and then a piece of wire outdoors to a grounding rod. This method does very little in the treatment of RF. Keep something in mind here, the NEC is concerned with safety and safety only, they couldn't care less about RF.
There should be one, and I say again, one, single point that you use for equipment RF grounding, my single point is the wing nut on the back of my tuner. From TT, purchase a TT-1251 RF Counterpoise Tuner, put it together and use it. From the wing nut on the back of the tuner run a short piece of wire or copper coated plumbers tape to the back of the marked terminal on the TT-1251, from the other marked terminal run a piece of RG-8 (length is not important) outdoors, only the center conductor of the RG-8 will connect to the TT-1251, outdoors you short the center conductor to the shield. Here you have two choices, you can terminate the RG-8 at an 8' grounding rod and see if that works to remove RF from the shack or you can run the RG-8 outdoors to a common tie point, at this common tie point install (buried) .15 wave length (one for 20m, 40m, 80m and 160m) pieces of insulated 14ga stranded wire at the ends of the wire solder them to a 2' or 3' piece of 1/2" copper tubing, hammer the copper tubing in the ground just below grass level. This last method is guaranteed to fix your RFI problem. Inside the shack, when tuning up set the TT-1251 for maximum current as shown on the meter. It will be necessary not to install the 470 ohm resistor that comes with the kit.
If you are still experiencing RFI after the above is implemented, there are other mods that can be made to the inside of the SDR to help.
Scott, I hope that this writing puts the RFI issue to bed once and for all.
References:
- ON4UN's book "Low Band DXing".
- Balanced and Unbalanced AC Power Systems. available on the Internet.
- Economical Antennas for Low and Medium Frequencies by Robert Wilson. A publication of Antennex Online Magazine.
Got more hints? Send me an e-mail!