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Author Topic: Front end protection in high rf environments....  (Read 5868 times)
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KD6VXI
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« on: June 24, 2017, 10:19:37 PM »

Question for the collective group.

Have a high power am station.  Capable of modulating a legal limit carrier. This runs into an inverted v mounted about 36 feet to the apex, ends about ten feet above ground.

That takes care of TX.

For rx, I have a few options, but one I want to play with is the following.

MFJ1026, fed with a pair of Homebrew 20 foot tip to tip dipoles. Since these will be used for lower bands, I have 20 dB amplifiers to put in front of them.  Each of the 2 dipoles will feed an amp, and each amp will input to the two channels of the 1026 (noise canceling / phasing box). 

How to protect the front ends of the amps and / or Mfj?

I thought a couple diodes in series, then back to back another set, at the feed point.  The amps can take a couple volt input.

This will / can cause bad rectification and retaliation, no?

I don't do CW, don't do break in, don't do ssb vox, and pretty much only operate ssb for actual DX.   My best bet a relay at each dipole that isolates the dipole and grounds the input of each dipole amp?



The Mfj and two dipoles for phasing seem necessary.  My noise floor increased fro S3 to S9 in the last year.  Two solar city optimized systems within line of site.

Any ideas I'm not thinking or seeing?

--Shane
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w4bfs
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« Reply #1 on: June 25, 2017, 09:49:28 AM »

hi Shane .... you are right in being concerned as many on this forum have tried multiple antennas and have rf burned gear...

I think you may need a switching scheme to avoid damage ... this would be derived from your ptt signal and properly sequenced...

I am interested as my 75 mtr noise floor is now about s9 as well.  73 John
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Steve - K4HX
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« Reply #2 on: June 25, 2017, 10:53:54 AM »

A few circuits that have been floating around a while.

http://www.ok1rr.com/index.php/technical-topics/39-yet-another-front-end-saver

http://www.ad5x.com/images/Articles/FrontEndProt.pdf


There are some commercially available items from the likes of KD9SV and DX Engineering too.
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KD6VXI
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« Reply #3 on: June 25, 2017, 02:45:54 PM »

Thanks for the replies and the links, Steve.

For those that are also using a SDRPlay, their is a metal box on eBay available. Really helped with noise pickup.

However you NEED to add ferrite chokes to help as well. 

I'll check the switching / sequencing circuits out.

--Shane
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W1AEX
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« Reply #4 on: June 25, 2017, 03:28:26 PM »

Hi Shane,

I use a 185 foot inverted v that is open-wire fed with the apex at around 60 feet and the ends at around 35 feet. I run full legal limit from 160 to 40 meters with that antenna and of course I was concerned about my two RX only antennas, which are 85 foot bi-directional K9AY type vertically-oriented triangular loops, each with a 24dB preamp at the feedpoint. One of the loops is about 120 feet from the transmit antenna but the other is only about 50 feet away. I wound 9:1 baluns with back-to-back IN914 diodes across the secondary, and installed them at the feedpoint of each loop and mounted my preamps a few inches away from the balun. I was a little skeptical that the 2N5109 transistors would survive the near-field RF as quite a bit of RF will pass through before the back-to-back diodes go into conduction, but after 5 years of this abuse they are still working fine. Even more surprising is that the diodes have not failed in either of the loops, in spite of the near field RF and numerous lightning storms.

As a noise mitigating measure I wound 1:1 transformers and installed them at the window threshold where both RG-6 feed lines enter the house. From there each RG-6 feed line is run into a relay box that disconnects the receiver(s) from the loops and grounds the center connection of each feedline (on the loop side) whenever my T/R line is keyed for any of the rigs. So far so good as I've had no receiver issues or failures.

I use the loops with my ANAN SDR for diversity reception (similar to what you plan with the 1026 box) or as stand-alone antennas with the ANAN or the SDRPlay for very quiet DX reception on the AM BCB and on 160/75/60 and 40 meters as well as SWL up through 14 mHz. The preamps actually extend reception higher than 20 meters but the Hex Beam does a better job up there.

Not sure if there is anything useful for you at my page but some of the details are here:    http://www.w1aex.com/loop/loop.html

Good luck with the project and 73,

Rob W1AEX

EDIT: As W1ITT mentioned, some sequencing is definitely needed. I made it simple and use a series of cascading relays to control the sequence. The system I use is pictured below and it has worked out fine.


* w1aex_cascade_relays_200d.jpg (50.74 KB, 1176x841 - viewed 326 times.)
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« Reply #5 on: June 25, 2017, 04:39:16 PM »

If you go the switching route, be aware of the timings involved.  Different relays have differing pull-in and drop-out times.  Simply controlling a relay, or set of relays, off your PTT circuit may or may not get you the protection that you need.  The EME (moonbounce) guys use sequencers to accomplish this and they know the timings of all relays in line as well as any delays purposely built into the sequencer. In absence of a spiffy timed sequencer, a bucket brigade of relays may get you where you need to go. 
Different from the days of hollow state  goodies, it takes but a millisecond or so to zorch a semiconductor.
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« Reply #6 on: June 25, 2017, 04:53:52 PM »

Then there are other "active" approaches.

I guess for a transceiver it's a different game to some extent.

But going back into time, Johnson (iirc) and Dow-Key (and others, including DIY) had a scheme that
used the characteristics of a pentode tube to bias it off during transmit, and act as a preamp during
receive. The things was called an "electronic T/R switch". I think the Johnson was said to create some
TVI problems, not sure. I used the Dow-Key for many years, think I still have it.

Well, the basic protection plan is that you really are not going to damage the front end of a receiver
with the output of a small tube... no matter what. As opposed to whatever RF gets picked up directly
through the air.

I've been toying with the idea of a "distribution" amp that would send signal to a number of receivers
simultaneously, and make it unnecessary to flip ant switches, etc.

I was thinking of simply parallel up a number of small tube followers to do that job, and doing the pick-off the same
way that the old electronic T/R switches did it... some of the ultra miniature "pencil" tubes will run fine
on very low voltages, like 12vdc or lower...  solid state front ends need a bit of extra care... but the diodes ought
to suffice... that's what I've been thinking.

Btw, W8JI says to not use the tube type electronic TR switch with solid state rigs, but it seems possible to
simply use the back to back diodes to limit any instantaneous voltage at the receiver... dunno.

* DOW KEY ELECTRONIC TR.pdf (50.6 KB - downloaded 275 times.)

* JOHNSON TR SWITCH.gif (38.57 KB, 1048x416 - viewed 368 times.)
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MikeKE0ZUinkcmo
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« Reply #7 on: June 25, 2017, 07:10:54 PM »

Quote
I've been toying with the idea of a "distribution" amp that would send signal to a number of receivers
simultaneously, and make it unnecessary to flip ant switches, etc.
I have used the Johnson T/R switches for a very long time, and they are an excellent solution to the antenna switching problem.

I have local broadcast station problems as well so I run the receive side of the antenna through a High pass filter.   The toroid in the T/R switch has terrible loss on the 160 meter band so I replaced it with a different mix.      


Here is the block diagram of my distribution system.





Mike

 
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Mike KE0ZU

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« Reply #8 on: June 26, 2017, 09:30:22 AM »


The last thing in Mike's sketch is a Mini Circuits unit.

It actually specs better on HF than the numbers shown, more like ~45db isolation below 15M, and
about >43dB above on 10M.

FYI.

* Mini Circuits ZFSC-8-1.pdf (213.54 KB - downloaded 237 times.)
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« Reply #9 on: June 27, 2017, 02:23:59 PM »

I got one on epay... the TNC to BNC adapters cost more than the unit!
HA!

More up there now, if anyone is interested... search on the "Zxxxx...." designator on the unit (pic below)

The chinese are selling Low Noise RF amps under $10 on epay, and of course they're not going to be as
good as the Mini Circuits, but I kinda doubt they need to be... Cheesy Going to start with one of them, and
see how I like it. Can always upgrade... will need an enclosure, but, them I've got.


* MICRO CIRCUITS 1-8 SPLITTER EBAY.jpg (227.57 KB, 1600x1200 - viewed 289 times.)
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