Any Ideas [AM BCB overload]



  • Thanks for all your replies. Quite a bit to read. I'm thinking I need to start from scratch when it comes to electronics. Like back in school we did work on breadboards etc, but that was 25 years ago. I'd like to be able to build my own solutions however I'll need to first learn what some of the symbology means and where / how to get the parts. I've found some spots already to buy but no overall kit to buy. Almost need to buy individually what you need. Would be ideal if there was some sort of plastic kit with an assortment of parts. I'm trying to run before I learn how to walk. I'm still reading my HAM basic book.

    I am moving the kiwi to our tower in a few months so it'll be much further away from 800/760 however it may be near some other stations so learning how to do this now would be in my best interests.

    Looks like most of the OV signals are in the -40's and I need to get them below -60 so yes I need about -20 on several specific channels. Funny part is I'm getting overload from a place in NORTH CAROLINA (Cypress Creek) which is 400 miles away! Unless there is a repeater somewhere.. I'll have to TDOA that to find out :)
  • I don't think -40 signals should cause an OV on the KiwiSDR. In my experience, once the signal is reduced to below -30dBm the KiwiSDR doesn't go into OV. If there is a signal approaching -20dBm or better anywhere between 0-30 MHz it will cause an OV problem. So usually it is the MW broadcast signals that are strong enough to do it.
    Ron - KA7U
  • But like I said my AM radio signals are -40's With a bandstop. I'm curious if I put 2 in a row if it would lower it further. I don't think so because it just impedes the same amount. Just a thought.
  • -40dB should be fine.

    So I think we now have to look for something else.


    Martin - G8JNJ
  • I think I'm going to get a telescoping fiberglass mast to test this. I'll connecting the ground straight down and a short wire to my laptop from my back yard and see if there is any overload. It'll be lower to the ground. Now to find some good telescoping masts for this sorta thing that doesnt conduct electricity.
  • Search "Spiderbeam 12m Fiberglass", that is designed for hoisting light antennas but a "10m fishing/flag" pole minus the two top sections will get it up a fair way and cost a lot less for a test setup.
  • 40ft wow. Nice. With a mini whip that should be fine since its so ultra light. Littly pricey at $105 USD but worth it if it works.
  • I think you may have to forgo the top section or two if you want the best stability and to be able to feed the coax down inside (I did that on my fishing pole, not sure of the internal diameters of the proper device).
    Place a decent earth at the base and suddenly you have a completely different beast in the E-Field probe. Here the noise is so bad I cut a small slot in the ground, lay the feed in that then drop a metal pipe (scaffold pole) over the top. Mine runs beside a raised (2" high) concrete path so it is easy to do that rather than wind RF chokes but in your case it may pay to assume there is pickup on the whole run and plan for dissipating that to ground and/or with chokes. If you use a feed through surge protection at the pole it makes working on the horizontal and vertical sections easier.
  • Max gain systems also sell heavy wall telescoping GRP tube ( I think they may supply MFJ)

    However I'm not sure that the support needs to be non-conductive, as the coax is anyway. If you connect the coax screen to a conductive support at both the top and the bottom I think it should work OK.

    Alternatively can you put the antenna in a tree or similar ? I have one suspended from a length of rope that passes over the limb of a convenient tree and it works fine.


    Martin - G8JNJ
  • edited August 2018
    This is interesting, was thinking to do the same once I am cleared of the local RFI and GPS power issues. Besides the RPA-1/2, what else is out there that'd getting use?

    Ha! I posted while reading the first page of the thread - I meant to ask about the preamp + equalizer strategy by @ka7oei and @G8JNJ and what other preamps out there exist within that same level of performance. As far as I could find a quick search, that's likely the best manufactured solution, and you'd have to build your own to best that.
  • Found the issue. Its power related. It said 5-12V and I was using a 12v and I wired the AC right in the box I think it was too much too close. Ran a direct power cable to it and 5v 1amp and no more OV tonight..
  • I've been able to get my mini whip on our tower but the performance is horrible. I kinda expected this with a mini whip because the cable has to be strapped to the tower and that tower is conductive. But I thought since it would be so high and so well grounded I may get some good range maybe some great signals in certain bands. But its horrible across the board. I looked at another person in my area and he's also chock full of noise. I have an AM bandstop on mine but I am getting a lot of repetitive signals across all bands. Can anyone take a quick look see what they think? Ethernet noise? There are some 5Ghz equip about 50ft higher up on the tower.
  • When you say that the performance of the Mini Whip on the tower is horrible, in what way do you mean that it is horrible?

    * * *

    There are a few common traps for those who use an active whip like the Mini Whip, including:

    - AM Broadcast signal overload. This would be indicated by hearing signals containing multiple AM stations' audio on frequencies on which they don't belong. If you live quite close to an AM broadcast stations - or there are several high-power stations within 10-20 miles (15-30km) - it may simply be overloading the whip's internal amplifier: This problem cannot be corrected by the addition of a filter in the coax between the whip and the receiver.

    I have built a mini-whip, but found it unusable at my home station because of strong AM BCB transmitters within 15 miles or so. I have also built similar whips with low-pass filters on the front end (between the whip itself and the amplifier) to remove AM BCB energy and solve this problem, but these devices are only useful for reception below approximately 500 kHz - which was the original intent. I have not attempted to engineer such a filter that would remove AM BCB energy but pass SW signals (e.g. an effective high-pass filter) nor do I know if it is even practical to do so.

    - Inadvertently bringing your noise sources out to your antenna via its feedline. This would be identified by hearing loud "buzzy" signals every 20-80 kHz, typically at frequencies below 1-2 MHz from switching power supplies, often accompanied by mains-hum modulated white noise at higher frequencies.

    I have long maintained that "most QRM begins at home" - and installing an "E field whip" like the Mini Whip is very good way to be reminded of this. The coaxial cable is connected in your shack to a receiver/power inserter which will inevitably be powered from the mains in your house - but those same mains are going to be heavily polluted with QRM from the electronic devices within your house (especially switching supplies which are in literally everything that plugs into a wall socket these days) and that noise will simply be conducted from the receiver, along the coaxial cable and to the whip, where it will easily get in. If you dare run your Kiwi and/or your antenna from a switching supply, you are just asking for trouble and headache!

    The easiest way to prevent this (after making sure that there aren't switching power supplies involved in your receive setup) is to put a common-mode choke on the coaxial cable. Typically, such a choke consists of many turns of small-diameter coaxial cable (RG-174 or, perhaps stranded RG-58) on a high-permeability ferrite core. For LF/MF use I have often used discarded TV flyback transformer cores, but a large "Mix 31" ferrite core wound with a dozen or more turns can help. This choke would go between the antenna and the receiver - preferably "close-ish" to the receiver - and the coaxial cable's shield (on the section between this choke and the antenna) would be connected to a "quiet" ground (e.g. not one that is tied to the house's electrical ground) to isolate the two at RF. It is common to also bond the coax shield at the tower as well. Note that the bonding of the coax to the "quiet" ground should be done to a solid earth ground with the receive coax looping down, very close to the ground before entering the structure - or before it parallels any other coax cables. (Coax and control cables that come from tower-mounted antennas that don't loop down to are bonded to a very solid ground before entering your house are sure-fire - pun intended - ways of bringing lightning into a house...)

    * * *

    Again, I'm not sure which of the above (if either) apply to you, but I've run across both (especially the second one) of them more times than I care to recall when someone installs a short whip antenna at their QTH.

  • Questions
    1. Is there metal above the mini whip?
    2. Is it earthed to the tower and through the coax? (try splitting it or one but not both)
    3. Do you have ferrite based CM chokes at the feed?
    4. Have you tried an earth at the base of the tower with more common mode chokes?

    I found here the vertical coax was feeding the copious electrical and local VHF noise into the whip, only lots of ferrite and earthing in two places on the feed seemed to give it a shot, never remotely quiet below about a couple of megs though. Best condition I got to was coax on the whip (with chokes) down to an earth rod then CAT5 cable and CCTV baluns (watch the polarity) back to the RX. I should have tried CAT5/Baluns all the way and I might do that with another whip I just bought to try.
  • I had it on my roof at about 1/4 the height it is now. Had 2x better reception. There may be an active AM station nearby. It is possible I'm closer here than I was at home.

    I thought you needed a switching power supply for the lna to work right? I do have a common mode choke on it, however it isn't grounded before entering the utility room. I've been wanting to do that. I'm getting the buzzy signals but I'll have to check the intervals. I did actually forget to put some ferrite cores on the power supplies which I'll go do now. Totally forgot about it.
  • edited March 2019
    Yes metal above. No the wire runs into the util room where it goes to the LNA which is grounded directly to the water main. So I should splice at the base of the tower and ground it to the tower as well as at the LNA..
  • Metal above the base of a E-field probe antenna is not a good idea. Pieter-Tjerk de Boer, PA3FWM, creator of WebSDR, explains why in this excellent article:
  • You need a nice old linear to feed a mini whip. Check it gives the required voltage at the right load, some 12V will be 18V off-load.

    There will be pick up on the feedline, it needs to be halted and/or fed to ground at the most affective spot(s), that is where the fun starts.
    At the very least see the outside of the coax as a likely source of trouble from common mode signals, break it up (in common mode terms) and ground it out.
  • Some of the 5GHz link kit I've had problems with in the past used Power Over Ethernet which was very noisy. In fact any POE kit seems to generate a lot of unwanted noise. This is a major issue with my Kernow KiWi which is currently using a temporary active monopole mounted above a room containing POE kit.

    Also some of the Bias Tees that are used with Mini Whips don't have adequate filtering on the DC path, so any noise picked up from the power supply or DC connecting cable gets transferred directly onto the RF path and into the receiver.

    However Mini-Whips and other E-Probe antennas tend to be noise magnets due to their unbalanced design and high value of feed point impedance, which makes it very difficult to choke off or completely eradicate noise that is picked up on the feed cable. Balanced active antennas and active loops (which have a much lower value of feed point impedance) are much better in this respect.


    Martin - G8JNJ
  • JKS yes so I'm limited as to what I can do for HF. I used this whip at home pretty well but it doesn't work well here. I knew it would be worse but not this bad. Any recommendations on anothe antenna to use? I won't want to use a g5rv because of the wires I'd need to hang from the tower down to the building. I'm trying to find something that will over at least from 80m up very well but can be placed on a tower with a standoff without looking too unsightly. I've looked at magnetic loops but a little too wide to mount to the side of this tower (its 140ft) I don't really have the space for this project to have too many antennas up there so I don't want one for 40M and one for 80M etc.
  • edited March 2019
    The LZ1AQ amp will work as a simple dipole, I think the balanced antenna may be easier to get working and I'm a real fan of CAT5/6 shieled feeds for RX antennas now.
    Mine is two loops with extra vertical sections and it is more sensitive in the vertical dipole mode but also more prone to the local noise.

    I also have an Aerial-51 Model 404-UL, that I keep meaning to put up somewhere (when I win the lottery) virtually invisible wire, they do heavier versions that cover 80m. The thin wire on the UL is probably not suited for permanent installation. I'm not sure if your wish to avoid the wire is visual impact or something else?
  • Yeah its a nice building with a tower but adding the extra wires would definitely be something the owner won't want.
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