A Question For Rich..

Increasing the OD of the 3-m Part 15 AM radiator reduces its reactance, and also reduces the rate at which that reactance changes when frequency changes. OD doesn't affect radiation resistance, as that is a function of electrical length.

Here are some numbers for 1700 kHz.

1/2" OD = 0.12 -j2500 ohms
8" OD = 0.12 -j945 ohms

The reactance change when going from 1700 to 1710 is 0.7% for the 1/2" radiator and 0.4% for the 8". It would be similar when going to 1690, but I didn't calculate it.

The lower reactance for the 8" OD radiator means it can be resonated with less reactance in the loading coil, which will reduce I^2R losses in the coil, and improve the radiation efficiency of the antenna system somewhat.

The rate of change in the reactance of the radiator with frequency, along with the Q of the coil sets the impedance bandwidth for the antenna system. So the 8" OD radiator can maintain a better Z match over more r-f bandwidth, and will radiate higher modulating frequencies better.

Any conducting path leading from the tx chassis to true r-f ground on/in the earth is part of the radiator, so the statements above apply equally to it, as well.

Hi Rich,

Do you think the effect of the 8" radiator on Z would be approximated by several parallel wires spaced about 8" apart in a cylindrical arrangement. Maybe held in place with wood or plastic spacers?

It would seem that the Z of each wire would combine in parallel lowering the equivalent Z. The unknown to me is the mutual coupling between the wires. I saw a ham antenna made this way but I don't recall the purpose.

Neil

Quote=radio8z: Do you think the effect of the 8" radiator on Z would be approximated by several parallel wires spaced about 8" apart in a cylindrical arrangement.

Yes, that approach should have similar results, ie, lower reactance & less dZ/dF. Obviously it also would have a lot less wind load than a solid structure of the same OD.

It could be modeled easily in NEC, but I haven't done it (so far).

The aluminum material I plan to use is sorta like a flexible straw. A 4' piece can be "stretched" out to a 8' length with ease. I plan to leave it somewhat compacted..

Would the total outside surface area (in length) be considered in the electricial length? There is space between the areas that are compacted..

"What Are These RF's and How Do They Do That..?"

Quote=12vman: The aluminum material I plan to use is sorta like a flexible straw. ... Would the total outside surface area (in length) be considered in the electricial length?

IMO the corrugations will have little affect on the electrical length of this structure at medium-wave frequencies, and you'd be safe just to measure it from end-end for an effective physical length.

What an FCC inspector might say about it I have no idea.

12vman,

I like your idea and willingness to experiment with this.

My experience with my indoor 1/2" pipe base loaded antenna is that it doesn't take much external influence (me getting near it) to spoil the resonance. I wonder if your corrugated pipe may bend in the wind and cause the same sort of detuning. Perhaps another benefit of lowering the Xc would be to make your antenna less sensitive to mechanical changes.

Just something to look for when you try it.

Neil
.

I figured on using P.V.C. pipe in the center for a framework to support the outside. A tee here and there and some of that insulating foam in a few spots to fill in the gaps. The material I have is somewhat rigid so I believe at ground level there won't be many issues..

LOL.. I know all about that tuning! Try tuning an antenna on a platform.. 8'x10'.. 20' off of the ground on a short step ladder..

My goal is to design an antenna that is broadband without the need of a "tapped" coil for tuning. The tuning can be done at a distance from the antenna to avoid such problems. No power for the tuning circuit required and it's center loaded..

Soon as the weather breaks, I plan to construct my ground area. I'll be using electric fence wire and at least 60 radials. If it doesn't go too bad, I may go for 120..

Isn't playin' fun.. 🙂

"What Are These RF's and How Do They Do That..?"