I am in the process of helping a friend in Oklahoma get on the air with his new General ticket. His dad (SK) had a 40 foot tower on the property with an unusual 'diapole' configuration that I have never heard of before and I was hoping someone on the forum could offer an explanation of what it might be. Picture a standard diapole with the balun at the top of the tower and one radiator of the diapole slanting and staked about 3~4 feet from the ground. On the other side, instead of one radiator there are two, taking off at approximately a 30 degree angle between them, again both being staked at 3~4 feet from the ground. Seems like this setup would exhibit some directional characteristics, I'm just not sure what they would be. He also mentioned that the single radiator appears to be longer than the two on the opposite side, maybe similar to an OCD. He's been up on the tower recently and didn't see a feedline choke on the coax, so not sure about that. The orientation is close to NNE/SSW. Anyone have any ideas about this? Thanks, 73 Chad KV4PD
"Dipole" It's some form of off center fed dipole. The balun eliminates the need for a feedline choke.
For starters, its "Dipole", not "Diapole". I'm surprised your spell checker didn't pick that up. Sounds to me like a Inverted-V. If the elements were horizontal, as in a dipole, it would show some directional characteristics perpendicular to the dipole wires. But when you start bring the element ends towards ground, the impedance is lowered and the antenna tends to be mostly omni-directional. You can actually get a very nice 50 Ohm match with a Inverted-V antenna. For it to be a OCD type, there would be a marked difference in the lengths of the wires on each side. Its possible that, the one side has two wires to increase the operational bandwidth. One wire cut for the high end of the band and the other for the low end. That might smooth out the SWR as you moved across the band. How long are each of the wires? What band is it intended for? What is the included angle, from one side to the other? The included angle should be around 90 degrees, but anything +/- 30 degrees of that will usually work. If the balun at the top is truely "Balanced to Unbalanced", there is no need for a feed line choke. A feed line choke is only needed if you have some feed line radiation and want to get rid of it. If there was a direct coax connection at the feed point, feed line radiation might be an issue. Then a feed line choke would be useful.
Oh, so that's what that red line under some words means.......duh. Dipole. I will have to gather more info on the parameters (distance from the tower base, etc) in order to answer your questions, but from what I do know at this point, it appears to be cut for 80 mtrs, albeit we just don't really know as like I said, his dad is a SK. He estimated the longest leg @ about 80 feet and the two shorter ones @ about 60 feet. I'll post again when I find out more details. Thanks for the replies. 73 Chad KV4PD
What you describe could work, it's effectively an off center fed antenna but one half has greater capacitive loading due to the two shorter wires. It's analagous to the way a quarter wave vertical can be tuned against a variety of different radial configurations or how you could build an off center fed antenna but then add loading in the form of a loading coil or a capacitive hat on one side to help bring the antenna back closer to a balanced condition. It would take some experimentation or modeling to find the combination of paired short wires vs long wire that resonates the antenna on a given band of interest. Of course this could also be driven with low loss line and tuned as a non resonant antenna. This is what it sounds like you're describing: And yes, if that's the antenna you're describing it would have some odd directional characteristics at least if used on higher bands. On low bands like 80m (assuming the elements were long enough to tune reasonably on 80m) it pretty much be an omni directional NVIS antenna with a very steep takeoff angle. Bottom line, you can load RF into an awful lot of antenna structures but the common forms like a dipole, quarter wave vertical, etc. typically have properties that folks have found useful like ease of matching or directional characteristics. But sure you can load up into bedsprings and window screens in a pinch to create an antenna. You can certainly load into something like what you describe if you deal with the matching issues but the resulting antenna pattern may not be the greatest. -Dave
There is two other possibilities that his dad may have had in mind: (1) The two wires opposite could be close to the same length which would have the effect of increasing bandwidth at the resonant frequency. (2) If the two wires opposite are differ in length so one is the same length as the opposite, and the other is 85 to 50% shorter or 30 to 40% longer, then the antenna may be dual band.
Wow, thanks for the graphic detail. Yes, that is pretty much what the antenna looks like based on a picture I saw yesterday. I got the tower-to-stake dimensions and ran the numbers to see just what length the radials really are and after allowing for less than total tower height (where the balun is hanging) and height above ground where the radials end, the results were surprisingly accurate for the 80~75 meter band. As K7MEM noted, "one wire cut for the high end of the band and the other for the low end", well that is exactly what I got. 3.503MHZ for the low end and 4.062MHZ for the high end. No doubt it will work for 80~75 and I'm going to guess that 40 will tune up within limits. He'll just have to try it and see what the results are for the other bands. Should be interesting. Again, thanks to everyone for their input on this. When Rob gets it up and running, I'll post the outcome. 73 Chad KV4PD
Hi Dave I believe you nailed it. Dad seemed to know what he was doing. Well he should have, being as they said on sesame street "The Royal Smart Person" ha ha. I am glad I am not as smart. Dad was sooo smart but lacked people skills, not putting him down, the world needs smart people and I loved him. Anyway, the antenna works great in all directions it seems and is good on 20 , 40 and 75 with auto tuner. Thanks for everyone's input and especially yours Dave. Rob. KG5RJD
Hi Dave I believe you nailed it. Dad seemed to know what he was doing. Well he should have, being as they said on sesame street "The Royal Smart Person" ha ha. I am glad I am not as smart. Dad was sooo smart but lacked people skills, not putting him down, the world needs smart people and I loved him. Anyway, the antenna works great in all directions it seems and is good on 20 , 40 and 75 with auto tuner. Thanks for everyone's input and especially yours Dave. Rob. KG5RJD
