Interference on Field day.

Discussion in 'General Technical Questions and Answers' started by N4JRP, May 26, 2017.

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  1. N4JRP

    N4JRP Ham Member QRZ Page

    In the past we have use two radios on field day. One on 20 and one on 40 but we always back down to one radio due to the fact of one radio interfering with the other. on transmit and receive. My question is is this a grounding issue? Should all radios be grounded at the same place? Or do I need some sort of RF filters?

    Thank in advance.

    Randall Phillips
  2. KV6O

    KV6O Ham Member QRZ Page

    Assuming that the interference you're getting is when on radio on one band is transmitting, and the other radio on the other band is receiving interference, this is a (generally) a filter issue - the RX radio is receiving LOTS of energy from the TX radio. Some radios are better at this than others - older radios with preselectors could mitigate this, dirty TX radios make the problem worse. What radios are you using?

    There are many solution out there, this is just one - specific band pass filters that limit the energy from the transmitting station to the band it's working in, or when receiving, limit the energy it sees from out of band transmitters. This will not fix the issues related to particularly dirty transmitters, hence the question about what radios you're using.
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    As posted above it's typically a case of nearby transmitters overloading receivers that lack the selectivity to reject the out of band energy. Solutions include:

    - Increased antenna separation and orientation to decrease coupling between the rig that's transmitting and receiving rigs.

    - Use of rigs that have better front end selectivity and ideally better strong signal handling. Older tube receivers tend to be very good at handling strong off frequency signals and receivers with tuned front end preselectors also do better than receivers with broadband front ends.

    - Use of outboard band specific filters to help reject strong out of band signals. IOW, hook up a 40m band filter to the 40m rig and 20m band filter to the 20m rig to provide additional signal rejection for the strong out of band signals. These filters either need to be hooked up to receive only antenna port (if your rigs support that feature) or need to be high power enough to handle the transmit power of the rig in question. IOW, if your rigs don't include a second, receive only, antenna input you'll want to use filters that can safely handle the transmit power out of the rig as in band filters rated for 100w or more.

    - Also avoid using receiver noise blankers if possible when operating high power rigs in close proximity to one another. Noise blankers effectively introduce a crystal detector early in the receive chain that is relatively broadband and can be susceptible to strong out of band signals. How big a problem this is depends on things like how much filtering exists before the NB stage and again how much signal coupling exists between the different antennas.

    It's almost certainly NOT a grounding issue.

  4. WA7DU

    WA7DU Ham Member QRZ Page

    Try moving antennas. If you are using horizontal wire antennas, run them perpendicular, or near perpendicular until you find the deepest null--elevation helps. Run one as a flat-top, one as a sloper. If you are using a vertical monopole, do not run a horizontal wire broadside to the vertical--run it lengthwise away from the vertical. For safety, keep the ends of dipoles out-of-the-reach of people and animals.

    This is not a grounding issue. But, take care with feed-lines running where common mode currents could be induced from one line to another.
    W4CDO likes this.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Doubt it has anything at all to do with "grounding."

    I've run many large FD stations and we never grounded anything since it was all temporary and only had to last 24 hours.

    No reason to shut off one station. I ran the K6CAB FD operation in the early 90s as a "15A" station (15 transmitters in operation simultaneously) and we had zero crossband interference. We also made the highest score ever set in FD up to that point (first station ever to break 30,000 points).


    1. Put up the best antennas you can, and keep them as far apart as possible. The FD rules allow for very wide antenna separation.

    2. If you have really good antennas, you don't need 100W of TX power to make lots and lots of contacts: Go QRP and every contact is worth more points, and this greatly reduces interference.

    3. If you can go QRP (good antennas help a lot with this), then also go "battery power," so each station can have its own battery. No AC power cords to conduct RF energy from station to station, as they're all completely isolated. Also "no generators running" makes a FD site quieter and smell-free, except for the grills cooking the great FD food!

    4. If you can't run QRP and really insist on using 100W rigs, use the "cleanest" rigs you can find. HF equipment varies enormously with respect to off-frequency transmission spectrum. SDRs are generally very clean, but then so are K3s, KX3s, and many rigs. Some are absolutely terrible and will create more crossband interference.

    Have fun and stay dry!
    KU4X likes this.
  6. N2SUB

    N2SUB Premium Subscriber QRZ Page

    Great suggestions.....

    It also might help to use the highest quality feed line as possible, and cover any antenna inputs on the radio that are not in use.

    Good luck.
  7. KV6O

    KV6O Ham Member QRZ Page

    Here's an article about running 2 radio on the same band (usually CW and SSB) and what it takes to do so. The main issue for the receiver is overload (or resistance to being overloaded), and phase noise for the transmitter - how much energy is being output outside of where it's desired. But the simplest solution in reducing these issues starts with separation - you have 1000 feet to work in. Inverse Square Law is your friend!
  8. WA9WVX

    WA9WVX Ham Member QRZ Page

    Here's some information on homebrewing BPF & Notches for operating a Kilowatt on HF. These are used at a Contest Station:

    If these operators use a KW while Contesting and not interfering with the other transceivers, you can imagine how well you should do using 100 W PEP.

  9. KW4TI

    KW4TI Ham Member QRZ Page

    One issue you may encounter is that the 40 m station produces harmonics for the 20 m station, especially when operating in the CW/digital part of the band. The easiest way to remove this is to use a shorted stub stop-band filter on the 40 m radio at 20 m. This consists of a half-wave of coax terminated in a short circuit applied at a "T" junction along the coax. A half-wave is the free-space half-wavelength (in this case 10 m) multiplied by the velocity factor, so for example for RG-58 it is 0.66, so you would use 6.6 m of coax cable. In practice you start with a bit more and trim it down. It helps to have an antenna analyzer when you do this. Because the half-wave stub is a quarter wave at 40 m, it transforms the short circuit to an open circuit so that it should not affect the 40 m operation.

    Here is a picture of the shorted stub circuit and transmission


    Here is a double stub filter


    If you have some coax tees and some coax this should be pretty easy to put together.

    Last edited: May 26, 2017
  10. N4JRP

    N4JRP Ham Member QRZ Page

    Wow Guys! Thanks for all the help. I have a lot of information to digest. Thanks again!!!!!!!!!!!

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