Please excuse the barrage of questions...
Don't worry - you are asking all the right questions! I see the gears turning..
NOTE: I have built the suggested 4’ and 1.5’ OCFD, and I am currently testing it. Initial observation is I have lost some reception at 155MHz...
Yep - this multiband antenna isn't really optimized for any single frequency except for 88 mhz - and relies on harmonics and the offset to give a compromize, so no surprise there. BUT, evaluate it from a SYSTEM standpoint: is the reception at 155 mhz good enough? How is the reception at 460 and 880 mhz? They may not be optimal, but if you are picking up the targets you need, this may be a keeper - or something to throw in the closet while you build the other ones for comparison to it!
The problem with using the harmonic trick is that although your receiver and transmission line may love it, the real-world angles that the antenna is looking at might be very much straight up, rather than looking outwards. In other words, when you start to make an antenna longer than the dipole length (half-wave) then unless you compensate for it, the antenna, if it had eyes, would be looking way above the horizon - this may not be desirable.
Also don't forget that you are feeding this large broadband antenna into a handheld - so on the 155 mhz frequencies, try using the attenuator as a test. If the signals improve, we are overloading the front end. If they just get weaker, then we know we have to make a better antenna.
In fact, if you listen to the WX channels, does attenuation have any affect at all?
And for that matter, what does the 4 feet and 1.5 feet off center dipole equate too?
Well, not to the usual 1/3 point, although it appears close. Since the bands we are interested in are not exact multiples of even harmonics, it was fudged a little bit to be a little more broadband - but we still have to deal with the antenna reception lobe patterns looking upwards instead of outwards as the frequency rises above 88 mhz.
In other words, this OCFD that you built is about as good as it is going to get. Might be time to shelve it temporarily, unless you are satisfied with the *overall* performance, compromised as it might be.
I cannot figure out how to calculate for the off center, of any wave length. Any help with this?
The easiest way would be to calculate for a regular half-wave (468 / F mhz). Then, multiply that length by .33. This is where you would cut the half-wave element in two and attach the balun. BUT this will only put you on the primary freq and even harmonics, and there is much in between. But there you go.
Would it be then, a 1/2 off center fed dipole for 155MHz would have a total length of 3.02 feet, but one leg would be 1.01 feet and the other leg would be 2.01 feet?
Yes, but the ocfd especially with a 1/3 offset responds primarily to the even harmonics, so.... 155 * 2 = 310 mhz. So maybe if you wanted to build a VHF / Mil-air dual-bander.
The cool thing is, you can now do things like this, and not wait for a manufacturer to come up with something!
Center-Fed Dipole = primary and odd harmonics
OCFD = primary and even harmonics (except for the 6th). Fudged for scanner broadbanding.
Look angles rise above the horizon as the harmonic frequencies get higher.
I realize now also that a 1/2 wave Center Fed dipole is two 1/4 wave legs together.
And is really optimized for one band. Fortunately, a center-fed dipole is 73 ohms or thereabouts, and is a great match to a 75 ohm transmission line. Thus you don't need a balun.
You could grab an "F-Chassis" connector or Radio Shack "Panel Mount F-61 Jack #278-212" to make it easy to connect a dipole directly to the RG-6. Connect one wire under the nut, and solder the other wire to the center conductor and bend them vertically. Make sure you don't short the center pin to the connector shell if you bend it. These connectors are pretty delicate, so indoor / temporary mounts are the rule of the day here.
NOTE: This chassis connector trick is extremely useful for making simple verticals with radials - keep that in mind!
I can grasp the concept of a center fed dipole. For instance, a 1/2 wave center fed dipole for 155 MHZ would have a total length of 3.02 feet and each leg of the dipole would be 1.51 feet. Now, how would I convert that into an off center fed dipole?
Now we're getting somewhere. If you built a center-fed dipole for 155 mhz (no balun - just a direct connection) - it responds to ODD harmonics as well as the primary obviously. There is no need to go off-center now.
Center-Fed Dipole = odd harmonics
OCFD = even harmonics (except for the 6th). Fudged for scanner broadbanding.
Look angle rises as the frequency rises above the primary freq.
So guess what - 155 * 3 = 465! A simple center-fed dipole cut for 155 mhz would at least take care of TWO of your bands of interest in the simplest way possible. However, we are dealing with harmonics again, and at 465 mhz, the antenna would be looking upwards instead of outwards, unlike at 155 mhz primary, which would look outwards across the horizon. How much difference? At 155 mhz, it would be zero degrees across the horizon. (ideally). At 465 mhz, the antenna looks up at about 45 degrees with this setup. This could be a deal breaker.
But would it be ok? Simple to try! Just cut your dipole, (about what - 18.2 inches on each wire) attach it to the F-chassis connector with NO balun, and check it out. Even 866 mhz *might* be good if signals are strong, even though it isn't optimize for 866 mhz.
Getting closer - you could even build a "fan dipole" at this stage to get 866 in better. On each side of the dipole, add a 3.25 inch wire and spread it apart from your original wires a little bit - maybe at 20 degree angle or so.
But what to do with that useless balun now? Well, it will do just fine if you decide to try out a "folded dipole". This is center-fed on one side, but is really a very skinny rectangular loop with 300 ohm impedance. It is broadband on the band of interest only - much like a fat copper pipe would be - but we aren't going there now.
I think that if you opt for putting the ocfd away for now, start another thread say for the fan dipole so we can keep track of your results better...
