40 - 10m Offset Dipole with 4:1 Balun

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by 2E0DEP, Nov 26, 2021.

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  1. 2E0DEP

    2E0DEP Ham Member QRZ Page

    Hi, today I received and installed the above named antenna.
    It’s in my loft space, the ends are bent round by a meter or so as I didn’t have the length in my property to keep in a straight line.
    It is supported by cable-tie’s.
    It’s connected via RG213 coax of about 5 meters.

    Upon connecting to my FT-897D and FC30 atu the radio comes alive…. all good so far.

    I first auto tuned 40m, it worked ok although the swr was just below 3:1 20m tuned up, with swr about 2.5:1, 10 meters and was the best swr readings although still about 2:1.

    I don’t know what happened but a short while later the swr on 40m has gone above 3 and the atu refused to match it.

    What could have happened?
    Don’t these antennas like being indoors?
    I think I read somewhere that cable-tie’s affect it?
    Where is the best place to erect it?

    Using a Multi-meter I selected the continuity setting and tested the newly soldered pl259’s, they were fine disconnected but with one end attached to the balun and probing the other end it suggests there’s a short circuit??
    I removed the end on the balun and probed the outer and centre of the so239 and it suggests there is a short, is this normal?


    Attached Files:

  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    No antenna really 'likes' being indoors. Sometimes you can get away with it but even in the best case an antenna outdoors and in the clear is almost always better than an indoor antenna. When you do install antennas indoors they often tune 'long' due to capacitive end coupling between the antenna ends and things like wooden studs or metal objects in the attic and that can be worse when you have to fold the ends of the antenna to fit the space and those folded ends run close to parts of the house.

    The 4:1 balun on your OCFD is almost certainly a voltage balun which will read as a short circuit on an ohm meter so that meter reading in and of itself isn't a problem. From your description I'd guess you overheated the core in the voltage balun, either that or some part of the insulation broke down and there is an actual RF short (different than a DC short measured with an ohm meter).

    The best place to erect any antenna is outside but for some hams that's not possible but if possible getting the antenna outside and in the clear can resolve a lot of problems including high receive noise levels or issues with RFI into household electronics.
    SWL37632 and 2E0DEP like this.
  3. 2E0DEP

    2E0DEP Ham Member QRZ Page

    Thanks K7TRF for your reply.

    Overheated the core?? Because of my license class I’m restricted to a max output of 50 watts, at the time of use the radio was set on 30 watts.

    Can RF shorts be detected with a gadget, perhaps a multimeter? (the multimeter is all I have atm)
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    If an OCFD is fed through a 1:4 transformer, the operation of that dipole depends strongly on the type of transformer. A 1:4 transformer means that it will be placed close to 1/3 of the way along the dipole.

    If the transformer is a "voltage mode" (auto-transformer) type, then it provides almost zero intrinsic blocking of Common Mode Current on the coax shield below the dipole and the section of coax between the transformer and the rig becomes an active, radiating part of the antenna. You do not want that to happen if the coax runs through the house and the antenna is in the attic. When used for receiving, the coax is close to RFI producers that are then received.

    If the transformer is "current mode" (Guanella two-independent ferrite cores type, or an auto-transformer co-located with a ferrite core coax CM choke), then the CM is blocked. This minimizes (if not eliminates) the CM current along the coax shield between the attic and rig, lowering the chance that RF will interfere with some house appliance during transmit, or that you will receive RFI from those appliances.

    I strongly suspect that the OHSF-40 is of the un-choked, auto-transformer type, and you will have lots of problems with it. Only opening the box would tell us how it is made.
    2E0DEP and SWL37632 like this.
  5. W1VT

    W1VT Ham Member QRZ Page

    It is quite possible to damage a "400W" core with 30 watts. Ferrite impedance matching devices can be quite efficient when everything is done exactly right.
    Jerry Sevick wrote a book that described many optimized designs. Relatively small cores that could handle a lot of power. He used the equipment he had at work.

    A moderately large 2.4 inch diameter core can only get rid of about 10 watts of heat. Ferrite is a relatively poor conductor of heat.
    Antenna designers make the limited power handling even worse by enclosing ferrite cores in plastic boxes.
    There are now cheap IR thermometers and relatively affordable IR cameras for those who want to measure core heating.

    I've taken up golf and found similar situation in that there are golfers who can hit the ball 300 yards when they do everything exactly right. Right down the middle of the fairway.
    Some of them spend a ton on lessons and fitted clubs so they can do it more often than once in a blue moon.

    Zak W1VT
    2E0DEP likes this.
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yeah, that doesn't sound like the most likely thing but your symptoms of SWR climbing as you use the antenna and then staying high is characteristic of an overheated balun core or insulation breakdown that led to a short somewhere. It could easily be something else but something gradually and then it seems permanently changed between the initial antenna install and now so the question is what changed?

    You might try disabling your rig's ATU and seeing what the natural SWR is at VERY LOW TRANSMIT POWER as in 5 watts or so. IOW, use your rig as a low power SWR meter to see how high the natural SWR of the antenna is without the ATU. If it's not down fairly low as in 8:1 or less (ideally a lot less) without the ATU helping the match then there's likely a substantial problem either with the antenna itself, how it's fed or the indoor installation like a shorted wire or it's much too long for the installed location or something along those lines.

    Is there any possibility of an outdoor antenna or is that simply out of the question?
    2E0DEP and SWL37632 like this.
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    About 25W if air in open air circulation. I design my transformers so that they are shaded from direct sun, but air can circulate freely around the ferrite.
    15W of continuous power dissipation in a 4"x4"x2" plastic box will cause the ferrite to reach Curie temperature. If you allow it to cool back to room temperature, the ferrite recovers as soon as it cools.

    I know, I have done the tests. I own one of these:
    2E0DEP and SWL37632 like this.
  8. W9IQ

    W9IQ Ham Member QRZ Page


    It could be that the antenna transformer overheated and melted some of the wire insulation. This would be surprising given its power rating but it seems few people design a proper transformer for this type of antenna. It could also be that there is a bad solder joint inside the transformer box or on the antenna itself that is causing the problem. Open the transformer box to inspect it and also double check all connections on the antenna itself. If you can post picture of the inside of the transformer box, that may be helpful for additional diagnostics.

    It could also be that your tuner has developed a problem. The MT-30 is minimally documented and specified but look it over for cold solder joints, burned wires on the inductors or even a failed relay. Also test the coax between your radio and the tuner for problems.

    It would be a good idea to get a proper common mode choke installed in the near future right below the unun on the antenna. This will help keep the coax from becoming part of the radiating antenna which can cause all sorts of frustrations.

    - Glenn W9IQ
    2E0DEP likes this.

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