Re: NEC-LIST: Unipoles and theoretical field strength

Alberto Fassio wrote:
>
> Hal Smith wrote:
> >
> > Dear Mr. Fassio,
> >
> > Has RAI any experience with practical field strength measurements of
> > operating unipoles?
>
> Hi Hal! We practically use only folded monopoles as they need less gnd
> plane, are protected against lightning and the measurements in the far
> field are practically the same as we had guyed towers.
>
> > I am a broadcast consultant and find that MW stations with folded
> > unipoles do not seem to perform as well as ones with series fed
> > antennae. Often they are used here when the ground conductivity is
> > not ideal so I'm not entirely certain of what is happening.
>
> It depends by the type of towers you have. First our metallic
> structures are not light as for example those proposed by Kintronic
> or LBA. In general we have big towers with a side of 2-3m.with very
> low losses. And in general they work with 10, 25, 50 or 100 kW. We
> have a kind of folded monopole of 175m with 1.2 MW!!
>
> > It happens there are three 1kW MW stations, all between 1400 and
> > 1490KHz here in the Oklahoma City area. Two of them use unipoles and
> > one is series fed. At the equidistant points (approximately 30km
> > distant) between them, the series fed antenna outperforms the
> > unipoles by about 50 percent. Not a scientific study but certainly
> > food for thought.
>
> It could depend by TXs or gnd characteristics or losses of poor
> structures (towers or cages) or poor gnd radials. To be investigate
> more in deep.
>
> Best regards
>
> Alberto
>
> > Sincerely,
> >
> > Hal Smith
> > AM/FM Services Company
> > Edmond, Oklahoma

Dear Alberto,

Thank you for your ideas on this subject.

The fact that RAI is using large towers with 2 to 3 meter face
diameter does give me good information. I have been wondering if the
metallic resistance of the tower itself could be a factor and it seems
that may be so.

Small 1 kW MW stations here in the US tend to use approximately 1/2
meter diameter towers. The towers usually aren't very well installed
or maintained either. I've had to have tower crews weld sections
together in the past, the towers should have been either welded or had
sections jumpered with copper cable when constructed but were not.

I am fairly certain about transmitter power for one of the stations. I
have measured its' driving impedence measured and I have measured base
current with a precision RF ammeter. I'm certain it is at exactly 1kW
input power to the antenna. But if there are ohmic losses in the tower
itself obviously the real efficiency would be less.

I've got mininec but I think I'm going to get EZNEC and buy the NEC4
engine for it so I can really get into all this.
Received on Sun Apr 26 1998 - 13:31:23 EDT