Simple Receiver Front End Protection Ciruit Idea

Discussion in 'General Technical Questions and Answers' started by N7BB, May 25, 2015.

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  1. N7BB

    N7BB Ham Member QRZ Page

    I want to add extra protection to my receiver when it's not in use. I have a transceiver and amplifier, and my receive antenna (a sloper) is in close proximity to my transmit antenna.

    Even though my receiver is ALWAYS off when I transmit, I don't always remember to disconnect the antenna.

    So, I found this simple circuit.

    ant-and-diodes.png

    I have some diodes lying around, & decided to construct a prototype. Note - the values of my diodes don't match what's in the diagram ( I can purchase those later).

    The idea is to switch the receiver to a stub of coax which terminates into an SO-239 with two diodes. Any stray RF should be blocked by these diodes. It's "extra protection" & perhaps a foolproof way of ensuring that, as long as I switch to this stub, any RF will not damage the front end of my receiver.

    thumb_IMG_0334_1024.jpg
    thumb_IMG_0335_1024.jpg
    thumb_IMG_0336_1024.jpg

    I realize it may be overkill, but I'm interested in at least knowing if I'm on the right track with the design here & what values of diodes would be appropriate.

    Or, did I simply construct nonsense?

    73

    Bryne
     
  2. K8ERV

    K8ERV Ham Member QRZ Page

    I think that is a common way, as long as the desired signal is not too strong, where the diodes conduct and cause intermod distortion.

    You can also use a relay contact to remove the ant and short the receiver input.

    TOM K8ERV Montrose Colo
     
  3. KO6WB

    KO6WB Premium Subscriber QRZ Page

    I'm curious what your receiver is?
    As for the diodes, in a perfect theoretical application, this would work too good and would short everything to ground.
    Good news, theoretical applications are just in theory. practical applications would dictate there to be a voltage drop on the diodes before they can go into conduction. That's sort of good news.
    The bad news is, the diodes you have shown above, need about 0.7v of forward voltage before they reach the point of conduction.
    Seems like a good number until you consider that's 700,000uV and your receiver would, or should, be overloaded way before that point.
    Still, it's an old trick that is used in a number of units but most are now using Schottky diodes or older geranium diodes. Both those have lower Vf then the silicon diodes. Still, it's a bunch to let into your frontend.

    Hope this helps
    73
    Gary
     
  4. AF6LJ

    AF6LJ Ham Member QRZ Page

    Keep in mind that if you live in a city or even a small town there is the likelihood that your ambient RF level across the hole of HF / LF might exceed the forward bias potential needed to make those diodes conduct, then you have a mixer...
     
  5. K9STH

    K9STH Ham Member Volunteer Moderator QRZ Page

    Diodes, across the antenna terminals, has been around for many decades. Unfortunately, when the received signals are strong, this produces all sorts of problems. As such, the radio manufacturers abandoned this a long time ago.

    Many "olde tyme" military receivers used a neon bulb across the antenna terminals (i.e. NE-2) to reduce the problem of transmitters causing damage. At a certain level, the bulb goes into conduction ("lights") and the r.f. is "dumped to ground". Some civilian receivers also incorporated this scheme.

    If you are really bothered by the possibility of damage, from a transmitter, then using a relay to short the antenna terminals, when transmitting, is really the answer.

    Glen, K9STH
     
  6. N0SYA

    N0SYA Ham Member QRZ Page

    I would use 1n4001s as they are hardier and also have a junction similarity to PIN diodes so they might make for less intermod than 1n4148s.
    Also see here for some input protection schemes;
    http://www.bobsamerica.com/preselect-6.html

    A PIN diode is almost useless as a rectifier of rf compared to a 4148 and other PN junctions, you don't want them to create noise in your rx protection ciurcuit, you want them to conduct when needed.
     
  7. WA7PRC

    WA7PRC Ham Member QRZ Page

    ^ ^ ^ THIS ^ ^ ^
     
  8. W9JEF

    W9JEF Ham Member QRZ Page

    On the recondx benches at AES, we had signal generators with fused output lines

    which prevented accidental attenuator damage when connected to transceivers.

    But a fuse might not act quick enough to prevent damage to a solid state front end.
     
  9. AF6LJ

    AF6LJ Ham Member QRZ Page

    Quite true regarding fuses and test gear.
    The fuses that are used for RF equipment protection are also very expensive.
     
  10. N7BB

    N7BB Ham Member QRZ Page

    The receiver is a Palstar R30A. The antenna is an alpha delta dx-swl.
     

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