Simple Dipole question

[WA2SQQ]

I have a simple antenna question that I’d like some opinions on. My 75m antenna is a shortened inverted V. Using two loading coils the overall length is 95 ft, 1.5:1 SWR points gives me a bandwidth of ~ 100 khz. The antenna is flat at 3880.

I’ve always constructed any dipole so that each side is of equal length. Last night I decided to try lengthening just the “hot” side of the dipole using clip leads of various lengths. I can easily lower the resonant frequency anywhere I want it, and the bandwidth remains the same.

I repeated the testing a second time, this time lengthening both sides. Exactly the same results. The end result suggests that both sides being exactly the same length may not be necessary. The “ground” side is effectively my ground plane. My goal is to come up with some way to remotely tune the antenna.

[W1AEX]

Assuming the dipole is directly fed with coax my guess would be that when the "hot" side is lengthened the imbalance causes the coax shield to become more involved as it becomes part of the antenna's radiating system. I would also guess that it probably affects the pattern somewhat as the shield becomes more of a radiator. That's not necessarily a bad thing if it works effectively for you. All of the above is based upon nothing but a guess that is fueled by my second cup of coffee this morning...

Rob W1AEX

[K1JJ]

I’ve always constructed any dipole so that each side is of equal length. Last night I decided to try lengthening just the “hot” side of the dipole using clip leads of various lengths. I can easily lower the resonant frequency anywhere I want it, and the bandwidth remains the same.

I would keep the dipole legs the same length, balanced.  It's usually best to do any job "textbook" all the way through. When every component is "textbook" there are no odd variables, rather, everything works together as it should and adds for maximum results. In this case, less feed line radiation - this can reduce a whole new set of problems.  Be sure to use ferrite cores and/or a hank of coax coiled up at the feedpoint as a choke balun

After doing it all textbook, if the overall performance is not right, only THEN start JSing around to see if things can be improved.

In your case, as you desire, put up a "perfect" dipole and use a tuner to improve its bandwidth match to the transmitter.

BTW, the loading coils are a great idea since you are using coax and need to get that center impedance up for a good match. The tuner will not help your coax losses, but will give the transmitter a nice match to work into.

T

[W3RSW]

As you move from center point the real feed impedance goes up. What little you did may have raised it from nominal 50 ohms to 60 or so.  Little effect with your small displacement.  That's the real part; the imaginary part varies too. 

As you might have known, a Windom, way off-center fed, may have an impedance of 200 ohms at fR in the classic example.

[WA2SQQ]

Understood. I'm only adding a total of about 11" to take it down to the CW section. Tonight's project will be to compare the complex impedance to see what is actually changing. The end effect will likely be insignificant for what I'm looking to do. Appreciate all the feedback.