Field Day Antennas and Interference between stations at the 100 watt level...

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KM3F, Jul 1, 2020.

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  1. KM3F

    KM3F Ham Member QRZ Page

    Lets hear from some who experience it year to year or otherwise, all the antenna configurations used, spacings per the site, Bands, Modes, operating Radio spacing, feed line spacing etc.
    I have a reason for asking to be revealed later in the thread, after looking at possible patterns that may contribute to the issue.
    Try to be clear as you can in descriptions and spacings.
    I know a lot field operations have this issue, so stay tuned.
  2. WN1MB

    WN1MB XML Subscriber QRZ Page

  3. N3FAA

    N3FAA XML Subscriber QRZ Page

    The only people who have this problem are the people who don't know what they're doing. Space your antennas out, run bandpass filters, and you'll be good to go.
    AI3V likes this.
  4. W9XMT

    W9XMT Ham Member QRZ Page

    If the definition detail assumed for "etc." includes it, r-f intermodulation between co-located transmitter+antenna systems can lead to the production and radiation of spurious signals in the r-f spectrum — which could cause interference to other, off-site services.
  5. KC8QVO

    KC8QVO Ham Member QRZ Page

    This happens quite regularly on Field Day.

    You need a set of these:

    Back in the day ICE used to be the go-to for these filters. For some reason I think they were bought up by DX Engineering, if not it was some other entity. So ICE is no longer around. I do believe the filters my buddy has are original ICE's for the contest bands. That was one of the best additions to Field Day that group ever did.

    If you are handy with making things you can home brew these filters also... They aren't rocket science, but do require some skill and a method of tuning (VNA, tracking generator on a service monitor, etc that can plot gain/loss over frequency spectrum).

    There are switchable bandpass filters also. See below for an example. If you are going to buy one for YOU then his is the route I would go. Then have the others do the same. If you plan on having a 3 station Field Day then having 3 of these would be a good idea. That way you aren't swapping filters around if you swap bands or if you have one station on sideband and another on CW on the same band at the same time you can do it.

    It looks like Array Solutions was the "other entity"... Their filters are very similar to the old ICE's. Here is a link to the 10m version:

    The AS-419 switchable filter box is the same model number as the original knob switchable version from ICE - the ICE 419.

    You can buy the W3NQN filters (individual ones like the 10m one above) in a set of 6 for the contest bands:

    And OM Power also makes a 6 band switchable box:

    Lotsa options.
    Last edited: Jul 1, 2020
  6. AA5MT

    AA5MT Ham Member QRZ Page

    We only have problems with qrm if we forget to install the choke baluns which keep the cables from radiating near each other.

    For single transmitter rfi, we also use a noise canceller, which is somewhat touchy to set up, but are extremely effective in cancellation. They work like noise cancelling headphones, and work on the same principal.

  7. W9XMT

    W9XMT Ham Member QRZ Page

    However r-f filters that pass an entire ham band and are used alone may not be able to reduce the generation and radiation of 2xF1 - F2 r-f intermods sufficiently enough not to interfere with off-site users of that spurious frequency.
  8. KW4TI

    KW4TI Ham Member QRZ Page

    A simple solution that can be made in a pinch is a coaxial cable stub filter to cancel a particular band because sometimes this problem happens and its too late to buy anything that specifically addresses this problem right before field day.

    Also placing your dipoles perpendicular to each others so they are polarized oppositely, or end-to-end so that the radiation pattern of one is in the null of the other, also helps reduce coupling between them. This is one reason why verticals or end-feds can be a poor choice for field day, because they are hard to keep coupling to each other.

    Another approach to consider is to use receiving loops. They can be tuned to be very frequency selective and turned to reject interference. Plus they are small and so easy to set up.
  9. KM3F

    KM3F Ham Member QRZ Page

    OK, here is the punch line.
    Two years in a row at the same site.
    75, 40, 20, 15 10 and 6 m.
    All verticals except the 75m and 6m were horizontal.
    Operating positions spaced out on separate tables.
    Antennas average 75 feet apart.
    Some of the feed lines even crossed one another.
    I check all of them for match, and set up two as verticals. 20 and 40m first year. Same 20m vertical this year.
    The 20m is my home made raised and is now 5 years old.
    Last year the 40m vertical was an Alum ladder with a top hat setup on a small utility trailer with radials and a Tuner at the base.
    This year the 40m vertical was usage of the 5BT antenna set up by it's owner.
    NO filters of any kind are used. No one even has any.
    Radios over the two years ranged from older Kenwood, Icom and Yeasu . TS2000, TS 430, 2x 746 Pro, FT897.
    No interference two years in a row and different operators.
    Not detectable interference from switching power supplies.
    No interference from the Generator.
    Your welcome to call it BS but that's our results.
    This thread was not about interference complaint but noticeable lack of it the second year in a row.
    Surprise. it did me the first year but the second time made it less much than a fluke.
    Other clubs in the area have a large issue.
    Why; I have thoughts but can't prove it. Suspect dipoles are one cause. Radiation coupling all over the place maybe another, common mode on the feed lines may all add to it, a lot of plastic in the radio cabinets that allow direct radiation, that we don't have all the effects from those possible causes..
    Call it luck the first time but something to it the second time.
    I have no reason to believe is will not be the same the next time after seeing it twice.
  10. AI3V

    AI3V Ham Member QRZ Page

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