[BC] RF fields around base of shunt fed monopole

[dynotherm at earthlink.net]

John,

If you accept Ron Rackley's configuration for a skirt/shunt excited
monopole, i.e. drape wires spaced a minimum of 5 feet out from the
sides of the central structure, attached at the 90 deg AM elevation,
dropped to a girdle wire and fed by a spider conveniently above ground
level at the bottom of the drape wires. When the spider Z is measured
and matched to the AM line with an ATU, the expected radiation is very
nearly (~98-99%) that expected from the same tower when series excited 
at its base in the more conventional way. However, with this otherwise
effective arrangement, I expect you will find a significant prohibited 
zone at ground level, probably exceeding that of a 90 deg base excited 
radiator due to the locations of the drape wires etc.

Generally, the performance of a NEC based model in these circumstances
may be less accurate than is desirable, but the model might be validated
by measuring the drape wire currents near the bottom, and by comparison
of measured drive point impedance vs. model prediction. Since a relatively
short tower with shunt excitation is under consideration, the induction
(current) fields can be expected to be the greater concern, especially 
since induction fields will be dispersed over a wider than normal area 
around the tower base by the drape wires. The real problem is you have 
no way of knowing exactly how much trouble you are in until you build.

When you look at the OET-65 safe occupancy for trained personnel in the 
model I expect you will find a problem with induction fields preventing 
close proximity to the area around the tower base, either in the model 
or in the actual build-out, or both.

Unless the weather operations can be remote controlled keeping people out of
tower base area, I suspect the station will have some dead air to sell in its
inventory, or perhaps they may wish to locate or build an alternate tower. 

One other possibility might be ground elevation via counterpoise with the
critical functional working area protected by Faraday cage below the RF
ground level. For AM use the tower will need a ground, either conventional
or elevated, so this solution may not be as strange as it sounds.

Phil Alexander, CSRE, AMD

-----Original Message-----
>From: John Lyles <jtml at losalamos.com>
>
>I have some questions for the collective:
>Looking at FCC OET-65 guidelines for RF exposure around 1 kW AM broadcast monopoles, it looks like the MININEC models suggest 1-3 meters separation from the base for a 1/10 to 1/4 wave tall structure at 1.5 MHz. This is the point where you would not exceed the magnetic and electric field allowable limits for controlled areas with knowledgeable workers. Has anyone actually verified these calculations around their tower base?
>
>Also, I imagine the model was a series fed, base driven monopole. What if shunt feed with a wire skirt - a folded unipole? Can i expect similar levels of RF around the circular wire that ties the vertical skirt wires together at the bottom, over to the ATU? In some cases, the ATU might consist of just a series capacitor to cancel the inductive part of the input Z. I would think that, depending on the complex impedance at the feed point of the tower, the fields could vary quite a bit, depending on the reactive components and stored energy.
>
>Local station is asking if they could co-locate at a weather tower, owned by the gov't. It would probably be shunt fed, 170 feet tall. The meteorologists are concerned about RF exposure as they work near the base often - once a week at least. They suggest shutting off the station each time. I could quote the 1-3 meter distance from OET-65 in my report but is this conservative or will the actual be grossly higher? 
>
>Thanks
>John Lyles
>