Hi new to this forum, hope my question is in the right place.

You may know the answer to this question.

When using a simple dipole antenna.

When rf current reaches the far end of a coax cable, some of it travels back down the outer of the coax unless a balun is used.
Does this mean that one element of a dipole ( the element connected directly to the coax outer) has less power flowing in it than the other? And am I correct in assuming that any rf hitting the balun is purely dissipated as heat?

My guess is that there is less rf power in one element than the other.?

Thanks in advance John G8HUY

 

Hello,

An open dipole is said to have an impedance of about 75 Ohms.
A folded dipole is said to have an impedance of about 300 Ohms.
Matching the cable to the dipole will have maximum power transfer.
An practical approach can be found in the following book an antennas:
https://archive.org/details/ARRL_Basic_Antennas

Bertus

 

Hi new to this forum, hope my question is in the right place.

You may know the answer to this question.

When using a simple dipole antenna.

When rf current reaches the far end of a coax cable, some of it travels back down the outer of the coax unless a balun is used.
Does this mean that one element of a dipole ( the element connected directly to the coax outer) has less power flowing in it than the other? And am I correct in assuming that any rf hitting the balun is purely dissipated as heat?

My guess is that there is less rf power in one element than the other.?

Thanks in advance John G8HUY

Yes, there is a radiated power implance on the dipole elements due to a current distribution mismatch on one side (coax ground) with an unbalanced feed. The RF balun is just a transformer so it doesn't dissipate the power from a imbalance. It changes the current distribution to a balanced configuration.
https://www.antenna-theory.com/definitions/balun.php

 

View attachment 284059

Yes, there is a radiated power implance on the dipole elements due to a current distribution mismatch on one side (coax ground) with an unbalanced feed. The RF balun is just a transformer so it doesn't dissipate the power from a imbalance. It changes the current distribution to a balanced configuration.
https://www.antenna-theory.com/definitions/balun.php

Thank you for your very helpfull response to my questions.

 

Thank you for your very helpfull response to my questions.

As a follow on to my query what are your thoughts about this dipole?
Will the rf still travel back down the outside of the coax? Where would be the best place for a balun to be fitted?
Once a balun is fitted would there be the equivalent rf power in both elements?
I do appreciate your ideas and suggestions. Thanks John

 

It's a form of sleeve antenna.

https://www.intechopen.com/chapters/17808

 

Sometimes happens that in one resonator of antenna is less current than in other. Then is used to say, the RF energy LEAKS via the braiding and very rapid someone is adding more ferrite cores on the cable or just adjust the balun or makes an another corrective actions, because without of repairing work becomes near impossible.

 

RE:G8HUY "both parts are 1/4 lambda"
The joke is that they are to be rather far from pure 1/4.
The upper part have rather thin wire thus probably the velocity factor is sth about 0.95 but it must be looked in tables according actual diameter the precise value. Secondly, very often there is no need to take the insulation off, thus the cable central dielectric is laid stay on. Then the V(f) is far below the 0.95, however higher than cable characteristic. Best method is to measure exact value of it. Third, the lower part comes with rather thick diameter, thus the v(f) value must be corrected according tables again. Logically that 0.5 mm and 12 mm conductors have measurably different velocity factors at least in the third digit or even at second digit.

 

RE: G8HUY ""what are your thoughts about this dipole?""

I liked it very much until found that it parameters are incredible bad.
Its completely impossible to stop RF leak via cable braiding down.
It is strong outer piping impact on resonant frequency, if no body pipe, the coaxial is too soft.
The antenna Z is never even not near the 50 Ohms.

The best results with this in range 156-182 MHz I got VSWR about 2. Worst examples had about 6.
Tried different cables, different sizes, plotted z by frequency - its incurable.
However one I use, the AIS antenna between sail mast and shrouds where it stays about 45 deg against mast in heigth about 3 to 4 meters. VSWR about 1.8 - best of best available. But any other antenna there would be more problematic, and power is small, thus, let it be so. But for main VHF station where 25-35 W are radiated that would be catastrophic.

As well tried it at 868 MHz. Results was again not plausible. I throw it out and substituted with 3 resonator Yagi. That worked very well. Tried J-Pole. Found it is ultra-critical about distance between loop sides. At loop width within order of 15-20 mm, small shake for +/- 0.1 mm changes VSWR from 1.05 to 3.7. Thats unacceptable.

Ah, ya. Just warning. The sleeve metal must be just polished as Brittish Queen`s throne. I made the Ground Plane for 152 MHz from aluminium pipes 11 mm staid about half century in the shelf. Slightly oxidized, but still mot area metal, and that white staff only at small areas. VSWR about 12. After I did everything I am able for, as the last chance I just slightly polished alu pipes and 12 fell to 4. Then I polished more carefully and got 2. Then I polished on the filtz rotator and got 1.2..... Thus make a verdict yourself.

Oh, grand thing are those P200/N1201SA and NanoVNA