Optimizing Part 15 AM Systems

[rfry]

Here is some information on this topic in response to an email I received.

An antenna system has a "feedpoint impedance" that is related to its electrical configuration. That impedance consists of its radiation resistance, and its reactance.

Electrically short radiators have very low radiation resistance, and very high capacitive reactance, resulting in extremely high input SWR. This means that almost no power can be delivered to it by a practical transmitter.

A loading coil can be used at the antenna feedpoint to "cancel" the capacitive reactance. However that also adds resistance to the antenna feedpoint. So then the antenna may have very low reactance, but much higher resistance than before.

Even if the reactance has been tuned to zero using the coil, the transmitter will not be able to deliver maximum power into it unless its output circuit can be tuned/adjusted for an optimum match into the final, net impedance existing then at the antenna feedpoint.

For example, if the antenna+loading coil had a feedpoint impedance of 50 ohms resistance and zero ohms reactance, but the transmitter circuits were optimized for a load with 2 ohms resistance and zero ohms reactance -- that is still a high mismatch to the transmitter, and it could not deliver much power into it.

So there are two steps to optimizing a Part 15 AM system.

First: Adjust the antenna loading coil for zero or near-zero reactance at the operating frequency.
Then: Adjust the matching circuits of the transmitter for maximum power transfer into that feedpoint impedance.

These two steps may have some interaction between them. And both of these steps would need to be re-done whenever a change is made to the antenna system (such as adding radials). A problem here is that measuring these parameters directly is expensive, and technically challenging.

Probably the most practical approach for Part 15 AM operators is to use trial and error to converge on the settings giving the greatest field strength, as seen on the signal level meter of a "ham" type communications receiver. The receiver should have little or no antenna on it, to keep the meter reading in the lower 1/3 of its scale.
//

 

[radiopilot]

> Here is some information on this topic in response to an
> email I received.
>
> An antenna system has a "feedpoint impedance" that is
> related to its electrical configuration. That impedance
> consists of its radiation resistance, and its reactance.
>
> Electrically short radiators have very low radiation
> resistance, and very high (capacitive) reactance, resulting
> in extremely high input SWR. This means that almost no
> power can be delivered to it by a practical transmitter.
>
> A loading coil can be used at the antenna feedpoint to
> "cancel" the capacitive reactance. However that also adds
> resistance to the antenna feedpoint. So then the antenna
> may have very low reactance, but much higher resistance than
> before.
>
> Even if the reactance has been tuned to zero using the coil,
> the transmitter will not be able to deliver maximum power
> into it unless its output circuit can be tuned/adjusted for
> an optimum match into the final, net impedance existing then
> at the antenna feedpoint.
>
> For example, if the antenna+loading coil had a feedpoint
> impedance of 50 ohms resistance and zero ohms reactance, but
> the transmitter circuits were optimized for a load with 2
> ohms resistance and zero ohms reactance -- that is still a
> high mismatch to the transmitter, and it could not deliver
> much power into it.
>
> So there are two steps to optimizing a Part 15 AM system.
>
> First: Adjust the antenna loading coil for zero or
> near-zero reactance at the operating frequency.
> Then: Adjust the matching circuits of the transmitter for
> maximum power transfer into that feedpoint impedance.
>
> These two steps may have some interaction between them. And
> both of these steps would need to be re-done whenever a
> change is made to the antenna system (such as adding
> radials). A problem here is that measuring these parameters
> directly is expensive, and technically challenging.
>
> Probably the most practical approach for Part 15 AM
> operators is to use trial and error to converge on the
> settings giving the greatest field strength, as seen on the
> signal level meter of a "ham" type communications receiver.
> The receiver should have little or no antenna on it, to keep
> the meter reading in the lower 1/3 of its scale.
> //
>


Rfry,

Good advice and the SStran AMT3000 which I use does just that, it would be nice if the creator of the AMT3000 transmitter would have provided an adjustable capacitor to adjust the capacitance seen at the ground, instead one has to solder capacitors to get the correct power transfer to the coil, I noticed this everytime I added new radials to the system, the match in the circuit would go out and I had to thereby increase the capacitance.

Adjusting the transmitters/loading coil is what we all do to get the most out of 100mw... but an SWR meter attached to 'read' a 100mw power is almost impossible at least with my MJF Deluxe Versa Tuner II (MJF-949E)....

Perhaps what we need is more to the dirty hands business end of it, say.... should the coil be wound with heavier gauge wire, right now we use 16 gauge wire for the coil, should it be 12, 10, 6, 4 gauge wire? We use PVC pipe for the coil form, should it be phenolic, nylon, delrin?, Most of use will install the transmitter just below the coil, is this practical, should it be placed much lower on the mast? Should there be a tuning capacitor from the unused end of the coil to ground, besides tuning the coil, would this benefit?

These are the types of 'practical' things we can do, as no one can measure the reactance/impedance of their coil properly, understanding the things we can do to improve our system will help much more...

Thanks.....

Radiopilot

 

[radio8z]

Re: Optimizing Part 15 AM Systems how about TIS?

Hi Rich,

Nice presentation.

I would like to ask you and others a question. I have seen three TIS systems in my city and have noticed that they all use center loaded antennas, yet the Part15 wisdom seems to be for base loaded antennas.

Any comments?

Neil

 

[rfry]

Re: Optimizing Part 15 AM Systems how about TIS?

> I have seen three TIS systems in my city and have noticed
> that they all use center loaded antennas, yet the Part15
> wisdom seems to be for base loaded antennas. Any comments?
_____________

Not a simple answer to this. The radiation efficiency of a center-loaded, short vertical can be higher than when base loaded, but the amount of difference also depends on ground system losses.

Here are the calculated radiation efficiencies of a 3-meter loaded vertical on 1.7 MHz for base and center coil locations, and two ground conditions.

For 20 ohm ground loss and center coil: 0.54%
For 20 ohm ground loss and base coil: 0.35%

For zero ground loss and center coil: 1.02%
For zero ground loss and base coil: 0.96%

So systems with low ground losses show less benefit from using a center coil.

A TIS antenna usually has a good radial ground system, and its ground losses probably are a small fraction of a typical Part 15 AM antenna system. But TIS antenna system designers may have some field experience showing that a center coil on their electrically longer radiators gives them a worthwhile benefit in some installations, and just decided to configure all their antennas that way.

Another factor here is that the inductance of center coils needed to resonate the short antenna system is greater than for base coils. This tends to increase the power losses in the center coil with respect to the base coil, which works against the expected increase in radiation efficiency from locating it in the center.
//