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AM band loading coil question.
AM band loading coil question.
I run a AM broadcast band transmitter part 15 legal for my back yard. I have a question about loading coils. I have been building MW antennas for some time now, so I have tried a lot of different types based on proven designs. I haven’t tried one idea yet, and before I start on it, I thought I would ask this consortium of broadcast professionals what their opinion may be. As you know, here are some variations of loading coil arrangements as to how the transmitter feeds the antenna. Most are wired so the hot lead of the coax goes into the bottom of the loading coil. That’s the basics; I know there are variations for tuning taps and things like that. I wonder what would happen if you had the loading coil attached to the antenna wire, the bottom of the loading coil would be grounded. All factors concerning loading coil inductance, antenna length, ect would be figured out so the antenna and loading coil standing alone would be resonant at the operating frequency. The feed from the transmitter would then be connected to the point at where the loading coil attaches to the antenna wire, not at the base of the loading coil as is usually done. The loading coil at one end would be grounded; the feed point would be at the top of the loading coil where the radiating element is attached. Since the loading coil and radiating element would be calculated to resonance, would feeding this arrangement at the top of the loading coil provide a proper match if the inductance of the loading coil would be variable to allow for fine tuning? The reason I am considering doing this is I am wondering if feeding the radiator in this arrangement would put more of the power into the radiator and cut loading coil losses. I have used this loading coil method to create a resonant antenna for applications using crystal radios, and it works very well, I haven’t tried it for a transmitting application though. I have used for crystal radio applications a coil in series with the antenna lead in also, however using a system where the base of the loading coil is grounded and the feed point to the input of the radio is on top of the loading coil works the best and gives me the best signal strength and sharpest tuning so I assume highest Q. If anyone has tried this loading coil arrangement for transmitting applications let me know how it worked out for you. Thank you,
Radio Joe
Total posts : 45366Joe,
There are some real experts on this topic here, myself not included, but here’s my thinking on this.
A series coil intended to resonate with the antenna will ideally present a resistive load to the transmitter. Whether the power delivered to the coil/antenna finds a return path depends on the transmitter design yet those I have seen do so through the circuit ground connected to earth ground.
But I think there is another way to think about this. My thought is to deliver the highest voltage possible to the radiator feedpoint and what you suggest should do so, but the practicality depends on the transmitter. According to my tests, the Ramsey AM-25 will feed any resistive load from 10 ohms to at least 100 ohms with the maximum power out at about 24 ohms resistive. Such is not the case with the SSTRAN AMT-3000 since it is designed to drive a 800 ohm load seen from the collector of the output transistor. It has been claimed that a series loading coil is the best approach for this unit since some of the coil inductance acting with the tuning cap. transforms the antenna system impedance to values acceptable for this unit.
I started my activities with the AM-25 and constructed a coil (about 79 turns #12 on a 1 1/2 PVC form. Across this coil I installed a parallel variable cap. in the 30 to 400 pF range. With the 3 meter radiator attached and “cold” end of the coil grounded I get a sharp resonance peak.
My feed is a 7 turn coil wound over the cold end of the coil (# turns worked best with the AM-25) with the cold end of the link grounded to the transmitter circuit ground only. I wanted to avoid ground loops at 60 Hz and this does so nicely. My antenna is mounted on the rafters of my basement (horizontal) and the ground is a copper water pipe. The results were too good!. I could hear my signal over two miles away on a car radio due possibly in part to the AM-25’s measured DC power into the final greatly exceeding 100 mW. For this and other reasons having to do with audio quality I switched to the SSTRAN unit.
The SSTRAN will “load up” and I get a resonant peak as gauged by using my scope as a field strength meter. I did find that I needed to keep R18 in the circuit to get good audio and unfortunately this robs some output power from the signal to the antenna system. This most likely indicates that my coils are not presenting the transmitter with the optimum load. My range now is about 500 feet using a car receiver and since this meets my objectives I have not attempted to engineer or maximize my antenna system characteristics. I have done no comparison between my antenna and the series coil design. Not lazy, it is just that it does what I want.
Hope this rather long post helps. I see no reason that your proposals would not work; you just may have to optimize the coupling depending on your transmitter. A link fed parallel resonant LC circuit is essentially a Tesla coil without feedback and will give a high voltage at the radiator feedpoint. I can light a NE-2 neon bulb by touching the radiator feedpoint so there’s some “juice” there.
Neil
Total posts : 45366I think that what Radio Joe is proposing is connecting the capacitance of the antenna in parallel with the loading coil, instead of in series. There is nothing wrong with this proposal in principle, but it presents considerable practical problems. The major problem is that the transmitter has to have a high-voltage output.
The function of a loading coil connected in series to the antenna capacitance is to supply high RF voltage to the antenna. At resonance, the voltage across the loading coil is quite high. For the parallel configuration to work, the RF voltage at the transmitter output has to be high. It is possible to design a transmitter with high voltage output, but the circuit required is a specialized design. I had discussed my experiments with a vacuum tube Part 15 AM transmitter circuit that produces a high output voltage on:
Total posts : 45366Neil said:
“Such is not the case with the SSTRAN AMT-3000 since it is designed to drive a 800 ohm load seen from the collector of the output transistor. It has been claimed that a series loading coil is the best approach for this unit since some of the coil inductance acting with the tuning cap. transforms the antenna system impedance to values acceptable for this unit.”The optimum load for the AMT3000 output transistor is indeed 800 ohms as Neil stated. But, this is irrelevant when looking at the entire base-loaded antenna configuration. The impedance at the bottom of the loading coil where the transmitter connects is anywhere from maybe 10 ohms up to over 100 ohms resistive depending on the quality of the antenna’s ground system. The AMT3000 transforms this low impedance up to the desired 800 ohm transistor load by means of an L-net. The L-net consists of capacitance to ground at the antenna jack followed by a small amount of series inductance (about 20 uH) between the antenna jack and the bottom of the loading coil. The capacitance to ground is the parallel combination of capacitors C23 and the trimmer capacitor. Where is the series inductance? It comes from part of the loading coil! This sounds strange, but this is common practice with loading coils and it is proven by the fact that the inductance of two series inductors is equal to the sum of the two inductances. It doesn’t make any difference if you connect a 20 uH inductor in series with the loading coil or let the 20 uH come from the much larger inductance of the loading coil.
The loading coil is an absolute requirement for the AMT3000 to work properly. If you just connect a resistor load across the antenna jack without a loading coil, you will indeed find that the optimum load is 800 ohms because there is no longer an L-net transfrormer in the circuit. But, this is not a valid configuration for the AMT3000, so it may be of interest, but is irrelevant to normal operation of the AMT3000.
The L-net capacitance from the parallel combination of C23 and the trimmer ranges from 572 pf to 660 pf depending on the trimmer capacitor setting. This range may not be adequate to cover the extreme high and low range of a user’s actual ground resistance. Optimum loading is indicated when 13 VDC is measured across the meter terminals T1 and T2 with no modulation. If the trimmer adjustment does not result in the optimum voltage, then the ground resistance is outside the “typical” range. This can be corrected by installing more radials or changing the value of C23. It is preferable to have a better ground, but for those who can’t achieve a better ground, then changing the value of C23 is a second option to at least get the proper impedance match.
Phil B
Total posts : 45366In response to Ermi’s post where he wrote:
There is nothing wrong with this proposal in principle, but it presents considerable practical problems. The major problem is that the transmitter has to have a high-voltage output.
That is the purpose of the link coil of a low number of turns I described which provides the voltage step up needed as well as the impedance step down from the high Z parallel resonant circuit back to the transmitter output. The same thing can be achieved by a tapped feed to the coil as Joe has suggested.
For PhilB, I think we both described the same thing, namely that a part of the series loading coil inductance functions with the caps. in the transmitter in order to affect the Z transformation. I suspect that the linked coil I described is not actually operating at resonance and is probably presenting the inductance necessary (at least to some degree) to allow the Z transformation with the SSTRAN. I did not tune the system to resonance, I just tuned for a peak field strength and I do not claim that my system is optimum but it works to my satisfaction in that it does what I intended, namely covers my house and yard quite nicely.
Anyway, thanks to both of you for your comments.
Neil
Total posts : 45366OK, after my last reply specifically about the AMT3000, here are some general comments relevant to Radio Joe’s original post.
Connecting the coax to the top of the loading coil where the antenna is connected will not work. Assuming you have a transmitter with a 50 ohm output impedance feeding 50 ohm coax, you will basically “short out” the loading coil. Ernie Ross said this in different words. The voltage across 50 ohms with 100 mW power is 2.24 VRMS. Typical voltage at the top of a loading coil at 100 mW is about 140 VRMS or more for a fairly efficient transmitter and coil.
The problem with base-loaded antennas is that the resonant resistance at the bottom of the coil is almost totally dependent on the ground system. The coil may introduce maybe 10 ohms, but the ground can range from an ideal of zero up to over 100 ohms, so the resistive impedance at the bottom of the coil would range from 10 ohms to 110 ohms or more. A 50 ohm feed at the bottom of the coil will be a mismatch to a varying extent. This is one reason not to use coax (aside from FCC rules).
The best overall transmitter/antenna arrangement for highest efficiency is to connect the output of the transmitter directly to the bottom of the loading coil with a short piece of wire (no coax) and to have a transmitter that has a means for adjusting the loading to match the actual antenna load (depends on ground quality).
If the transmitter load impedance is specified at say 50 ohms, then you can compensate for the lower than 50 ohm load resistance of a very well grounded antenna by feeding the coil a few turns up from the bottom and grounding the bottom of the coil. But if your ground resistance + coil loss is higher than 50 ohms, you have no option other than to improve your ground to get the total load down to 50 ohms.
Phil B
Total posts : 45366PhilB wrote The problem with base-loaded antennas is that the resonant resistance at the bottom of the coil is almost totally dependent on the ground system. The coil may introduce maybe 10 ohms, but the ground can range from an ideal of zero up to over 100 ohms, so the resistive impedance at the bottom of the coil would range from 10 ohms to 110 ohms or more. A 50 ohm feed at the bottom of the coil will be a mismatch to a varying extent.
Phil mentioned “resonant” resistance, and that resonance is an important concept. If the loading coil does not offset the capacitive reactance of the ~3-m antenna (resonate it), then the antenna system will accept/radiate little r-f power — even if the resistive term of the antenna system load impedance meets the value expected by the transmitter.
For example, a transmitter designed for a 50 ohm, non-reactive load will see an SWR of about 100:1 when driving a load impedance of 50 -j 1000 ohms, such as might occur with a mis-adjusted loading coil and a total ground/coil loss of 50 ohms. Even though the resistive term of this load impedance is 50 ohms, this antenna system will reflect 98% of the r-f voltage applied to it back to the transmitter output terminals.
OTOH if the load impedance in the above scenario was 100 ±j 0 ohms (ie, the system was resonant), the SWR would fall to 2:1, and the reflection would fall to 33.3%. Even though the resistive term of the load impedance is twice as high as the specification, these values would allow better performance than the non-resonant version.
So while the r-f resistance values of the coil and ground system are important, they are much less important than assuring that the antenna system is resonant.
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Total posts : 45366Radio Joe’s use of the word “coax” is confusing to me because Part 15 AM transmitter designs usually do not use coax, since any transmission line counts toward the 3 meter length budget in Section 15.219(b). I took Joe’s word “coax” to simply mean the transmitter output. Optional outdoor antennas using coax (for real-estate transmitters) have been discussed in this Forum, and their legality under 15.219(b) has been questioned.
In my previous post, I said that a high-voltage transmitter output is needed to drive the loading coil and antenna capacitance in parallel. To clarify, I will add that the transmitter output has to have high impedance, also. The transmitter output has to approximate a current source.
Using the loading coil in a step-up transformer configuration also increases the voltage across the loading coil, as Neil noted. A step-up transformer is used in the Rangemaster, for example. I was responding to whether the transmitter can be connected to the hot side of a grounded loading coil, so I did not mention transformers.
Total posts : 45366Ermi Roos wrote: The function of a loading coil connected in series to the antenna capacitance is to supply high RF voltage to the antenna. At resonance, the voltage across the loading coil is quite high. (etc)
_________A base loading coil is used to supply a reactance value at its output end that is equal in magnitude and opposite in sign to that of the electrically short radiator connected there. This results in a net load impedance at the input to the loading coil that is resonant at the feedpoint of the antenna system — ie, its reactance term there is zero ohms. This permits maximum power transfer from an r-f source (a transmitter) designed / adjusted for the purely resistive, net load impedance remaining there.
The r-f voltage for such a condition, for a given applied power, is relatively high at the output end of a Part 15 AM base-loading coil fed at its input. This is a consequence of physics related to the very low radiation resistance and high capacitive reactance of the electrically short radiator used (legally) on Part 15 AM systems.
But that doesn’t necessarily mean that any / every source such as a vacuum-tube transmitter providing an equally high r-f voltage will maximize the radiation efficiency of an electrically short radiator of a legal Part 15 AM antenna system without a loading coil to the same extent as with a properly selected / adjusted base loading coil.
To do that, such a high-voltage source would need to provide the same phase relationship between the r-f current and r-f voltage at the base of the 3-m radiator as exists at the output end of the appropriate loading coil for that electrically short radiator.
This is not a trivial condition to accomplish, without a loading coil.
Further comment is invited from all readers.
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Total posts : 45366A series-connected loading coil used in an antenna circuit with sufficiently high Q has a substantial voltage-multiplication effect at resonance. This high RF voltage causes a significant amount of current to flow through the antenna capacitance of the short antenna, and this capacitor current flows through the radiation resistance, which is in series with the antenna capacitance.
A loading coil connected in parallel with the antenna capacitance does not have a voltage-multiplication effect, and therefore the required high-voltage to get sufficient RF current through the antenna capacitance must come from the transmitter output. If the high-voltage RF source has a high impedance, a parallel loading coil is necessary to resonate with the antenna capacitance in order to provide a high-impedance load that does not significantly reduce the RF source voltage. The link in my post that Rich quotes goes to a part15.us Forum thread that (in part) describes my experiments with a vacuum-tube Part 15 AM transmitter that has a high impedance, high-voltage, source, with the loading coil and the antenna capacitance connected in parallel.
I have not attempted to make a high-voltage, low-impedance RF source, but if it were feasible to make such a source, driving the capacitance of a short antenna without a loading coil can be done. The phase difference between the voltage and current at the base of the antenna is near 90 degrees no matter whether a loading coil is used or not, because the antenna impedance is capacitive. A loading coil does not alter this phase relationship between voltage and current inside the antenna. To demonstrate this requires only ordinary circuit analysis. The following elementary example illustrates my point:
Just consider an ideal inductor in resonance with an ideal capacitor. In the inductor, the voltage leads the current by 90 degrees; and in the capacitor, the voltage lags the current by 90 degrees. In the series-resonant condition, the current through the inductor is in phase with the current through the capacitor, but the voltages across the inductor and capacitor cancel each other out. In the parallel-resonant condition, the voltage across the capacitor is in phase with the voltage across the inductor, but the currents in the inductor and capacitor cancel each other out. Drawing vector diagrams of what I described would enhance the understanding of what I am talking about.
Total posts : 45366Radio Joe,
There have been some good replies to your query (better and more polite than on the other board) so I wonder what you plan.
Neil
Total posts : 45366I think ill just connect it up and see what happens 🙂
Radio Joe
Total posts : 45366haha! thats the spirit!
🙂
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