Ham Radio Forum

I'm new to this blog. Perhaps someone can provide some help. I've got a 2kw non directional AM station at 1020kHz. We are a folded unipole, 3 skirt, with an antenna current of 20.6 ohms. We use a tee network. We have significantly improved the stations ground system with a 50' circumference of #10 soft drawn copper, to a copper strap frame, and interspersed the 50' radials with 120' radials. We also employ 9 above ground counterpoise radials with 4 of them close to 1/4 wave of 241'. We are limited by property dimension.

Heres my question. Prior to system changes, the former operator had far less power, but utilized an L circuit, a large tapped coil with a cap at the output. The stations coverage was very good. When we upgraded, engineers installed a Tee network, and coverage actually worsened.

Would we be wise to re-tap the vertical skirt for a near 40-50ohm impedance, and junk the Tee network in favor of the L circuit. I'd give anything to see this station's coverage blowout. The LPB transmitter likes what it see regarding VSWR, however going from 500watts to 2kw should have made a significant difference in far-field coverage. I do understand that as resistance increases with a folded unipole, so does reactance, but isn't that the purpose of the L network? Just seems to me that anytime additional coils, taps and connections come between the transmitter and antenna, there are bound to be losses, especially in a seashore, high salt concentration environment.

Any suggestions would be very appreciated.

1) What is the height of the folded unipole?

2) I assume it is composed of a vertical tower with a side down-conductor of some kind. Is the down-conductor copper? What is the down-conductor cross-section diameter or wire size?

3) Are the 50' and 120' radials buried or lying on the surface?

4) Does the copper strap frame circle around the base of the unipole? If so, what is the distance from the unipole base to the copper strap frame and what kind of connection is there between the tower base and the copper strap frame?

5) Is the T network composed of two series inductors with a capacitor to ground from their center connection point or two series capacitors with an inductor to ground from their center connection point?

6) Are all three T network components adjustable?

7) I assume that the transmission-line from the transmitter is 50-ohm hard line. Is that true?

8) You stated that the antenna current is 20.6 ohms, but obviously meant the antenna base impedance is 20.6 ohms. What is the antenna base current? What is the current into the transmission line from the transmitter?

9) If there is any time during the week when the station signs off the air so you can check, do any of the antenna tuning unit components run excessively warm. (Slightly warm may be normal.)

-Bob

Thanks Bob:

I really appreciate your help. Yes, you are correct. The Antenna base impedance is 20.6 ohms. The Folded Unipole is mounted on a grounded tower 306' tower, the total height of the three skirt wire unipole is 208'
and is offset from the tower with PVC pipe 30" from the tower itself.

The unipole is comprised of #6 stranded copper, and yes, we strapped the base of the tower with 4inch copper strap, cad-welded to the tower itself. The DISTANCE from the bottom of the folded unipole's Bottom, at the "Hot ring" to the ground itself is 20'. The 4 inch copper strap serves as the start point for the ground radials which have just been installed and will be covered with 2 inches of soil. I am not sure why the unipole was placed 20' off the ground, but thats where it is. A 1/2inch, 17' piece of refrigerant copper feeds the hot-ring from the bowl on the ATU. Approximately 7" BELOW the hot-ring is a copper ring fixed to the tower, and it is from there that the counterpoise radials stretch to 20' poles at the property boundaries. In all there are nine counterpoise radials made of copper from 225' in length to 140' in length. Again, the total distance from the hot-ring at the bottom of the vertical skirts to the ground is 20'.

The Tee network is made of the T network composed of two series inductors with a capacitor to ground from their center connection point.

Bob, the Tee network utilizes a 500 pF glass Jennings vacuum capacitor at the Tees output. We are using 7/8 foam hardline from the transmitter with a 50-ohm load from the transmitter.

Thanks so much for your help. I have shut the station off and have discovered that while the caps in the Tee are cool, several times I've discovered high temperatures at the clips on at least two of the three coils. I've had to clean them well, and then, they operate normally cool.

This facility is capable doing much more than it is currently doing, and even though we have a truncated ground system, it is widely believed that our Q or efficiency should be improved substantially. Any thoughts you might have would be greatly appreciated

Thanks

This doesn't seem like an antenna design that the FCC would be apt to accept for AM Broadcast service. Is the station located in the United States?

It wouldn't think PVC pipe would be strong enough to hold the 6-gauge vertical wires. Are you sure that the supporting pipes are made of PVC?

If I understand correctly, the counterpoise ground wires connect 20 feet above ground radials that will be buried. There will be phase shift between the two ground systems if they are merely connected together and ground losses will be higher than necessary. It is not common to use both ground radials and a counterpoise, but they are sometimes used together. Where both are used, the counterpoise system normally is driven such that if it is assumed that the earth is in a neutral plane, the counterpoise has the opposite polarity with respect to ground, as does the antenna. In other words, when the antenna voltage swings positive with respect to earth, the counterpoise voltage swings negative with respect to earth, and vice versa. That requires a more complicated antenna tuning unit than the T network you currently have. There will be distribution of current between the buried ground system and the counterpoise that will depend upon the negativeness of the counterpoise with respect to ground and there will be a particular division of current for your two ground systems that will result in minimum total ground loss.

For optimum effectiveness, the spacing between counterpoise wires should not be greater than their height above ground and they should extend out from the antenna a distance at least equal to the height of the antenna above ground, and preferably twice as far, so they will intercept most of the electric field in the vicinity of the antenna.

Was the 20.6 ohm antenna base impedance measured or calculated?

Is the antenna base impedance resistive or partly reactive? If partly reactive, do you know the resistive and reactive components?

What is the VSWR at the transmitter feed point?

What are the values of RF current at the transmitter feed point and at the base of the antenna?

If the station is located in the United States, what 1-mile attenuated field strengths were measured in each of the eight or more radials required by the FCC during the last Proof-of-Performance measurements and what 1-mile unattenuated field strengths were calculated from those measurements? Have 1-mile attenuated field strength measurements been repeated since the power, tuning unit, and ground system changes? If so, what are the new measurements and new calculated unattenuated field strengths?

-Bob