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Hello,
I made a loading coil chart for a 10 foot broadcast AM vertical antenna. So far, i made a chart between 1360 and 1710 kHz. Frequency lower than 1360 kHz is useless because of the lower radiating efficiency.
http://www7.brinkster.com/yvesroy/10ft-antenna.asp
Any suggestions and comments are welcome.
Yves
Hello,
I made a loading coil chart for a 10 foot broadcast AM vertical antenna. So far, i made a chart between 1360 and 1710 kHz. Frequency lower than 1360 kHz is useless because of the lower radiating efficiency.
http://www7.brinkster.com/yvesroy/10ft-antenna.asp
Any suggestions and comments are welcome.
Yves
Total posts : 45366YvesRoy,
What would be really helpful, for most folks, is a chart for a 102 inch whip antenna. If you are up for more research and testing, you might be a real hero for many.
Good work on the chart.
Marshall Johnson, Sr.
Rhema Radio – The Word In Worship
http://www.rhemaradio.org
Total posts : 45366Hello Marshall,
Can you give me the diameter of your 102 inch whip antenna? Maybe i could do a second chart for this whip antenna.
Yves Roy
Total posts : 45366Yves,
The average overall diameter is approx. 3/16″. The diameter is non-uniform being about a 1/16″ thicker at the base. The base has a threaded mount bolt approx 5/8″ long and a 1/4″ static ball at the tip. Good luck.
Marshall Johnson, Sr.
Rhema Radio – The Word In Worship
http://www.rhemaradio.org
Total posts : 45366[quote=YvesRoy]Can you give me the diameter of your 102 inch whip antenna? Maybe i could do a second chart for this whip antenna.[/quote]
Just to note that for a given operating frequency, reducing the average diameter of the 3-meter-long section of radiator attached to a Part 15 AM tx at its “antenna output” terminals will increase the capacitive reactance of that radiator, and at the same time reduce its SWR bandwidth (eg, it will increase the “Q” of that radiator).This means that the X(sub L) of the loading coil needed at the feedpoint at the base of that antenna to bring it to resonance will increase. Typically that increases the r-f losses in that coil, and so lowers the radiation efficiency of that antenna configuration.
So with other things equal, the smaller the average diameter of the radiating structures (including those of the “ground lead,” “ground conductor,” and/or “ground wire,”) the less will be the overall radiation efficiency and r-f bandwidth of that radiating system.
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Total posts : 45366So it seems that, within reason, larger diameter radiating structures are preferred?
Any other dimensional relationships that have a positive or negative impact?
Larger or smaller diameter loading coil?
Space between windings on a loading coil?
Using a toroid instead of an air coil?
I could probably find some of these by going back through Rich’s and others posts, but if anyone would care to post an “all together” summary 🙂
Total posts : 45366Hello Rich,
I’m using a 3-meter long copper pipe with an 0.5 inch diameter. Based on my calculation, there is some improvement about radiating efficiency if i use a big copper pipe with 1.5 inch instead of 0.5 inch.
For 1500 kHz, radiating efficiency is approximately around 0.50% with an 0.5 inch diameter of antenna. With an 1.5 inch diameter of antenna, radiating efficiency is up approximately to 0.60%.
Here in Quebec, a big copper pipe of 1.5 inch of 12 foot long costs around 40$ (35$ US). Remember folks, the length limit of the antenna from your transmitter is 10 foot.
Maybe i will buy a new big copper pipe to experiment this. I think that the main disadvantage is that the big pipe is not light at all.
Yves Roy
Total posts : 45366Balancing the Xl to Xc will bring the antenna to resonance but it does NOT match the impedance of the final amplifier/network to the antenna load. This impedance loading cannot be a wildcard when working with so little signal. Efficiencies only become worth considering when maximum power transfer occurs to the load (antenna). Resonance does not guarantee the proper operation of the antenna “system”.
And as an antenna designer, I usually use the length to diameter ratio to determine if the antenna conductor will be a useful transmission source. The fatter the antenna in relationship to the diameter the lower the Q-factor, the broader the pass band of the antenna. However, total surface area is still the trump card in improving efficiency. The real test of a quality AM antenna system is the true broadening of the bandwidth of the system allowing more high frequency audio sideband energy to be transferred by the antenna system over the air; this done with a high efficiency impedance and phase correction network. An almost infinite combination of capacitors and inductors of many types and varieties are available.
The 102 inch whip antenna is readily available and very resilient. Just like the uniform cross-section tower in licensed broadcasting, YOU make what you have work. Even 325 foot towers can be inefficient and exhibit a very narrow bandpass.
Some Part 15 transmitters simply ignore the impedance or bandpass of the antenna and install a shunt resistor in the output of the transmitter to keep it happy on a nine foot length of wire. The shunt resistor raises or lowers the load resistance on the transmitter without dealing with resonance or impedance matching of the radiator itself. This is not new thought. QRP amateur radio operators have been dealing with this for a long time.
Many solid state output amplifiers have an acceptable characteristic load impedance in the 5 to 15 ohm range, so a 50 ohm resonant load would seem to be the back door approach to a simple matching network which would control the load impedance, resonance, bandwidth and system power transfer matching and component efficiency all at the same time. This practice has been accomplished by licensed broadcasters for many decades. Solving complex impedance transformation can require exceptional skills. THIS IS EXACTLY WHY THE ANTENNA SYSTEM ON THE AM1000 RANGEMASTER IS SO SIMPLE AND EFFICIENT (and with a 102 inch whip antenna). The proof is in the performance.
That would allow one to conclude there are several combinations of capacitance and inductance that will maximize the antenna system. Broadcasters having been doing this with short towers/antennas for a long time as well. The new Kinstar antenna system provides new wrinkles to the traditional way of thinking about antennas. The quest for the best, most efficient antenna continues.
KEEP YOUR POWDER DRY.
Marshall Johnson, Sr.
Rhema Radio – The Word In Worship
http://www.rhemaradio.org
Total posts : 45366[quote=kk7cw]The fatter the antenna in relationship to the diameter the lower the Q-factor, the broader the pass band of the antenna. However, total surface area is still the trump card in improving efficiency.[/quote]
As an example to clarify the above statement about Part 15 AM antenna efficiency, a linear radiator comprised of a 3-meter length of #28 wire by itself has almost the identical radiation efficiency as a 3-meter length of 6″ diameter conductor, when both are resonant and equal r-f currents flow in both of them.The benefit of the larger diameter radiator is that its input capacitance will be lower, and more constant across the operating bandwidth.
The lower input capacitance means that the loading coil needed to resonate that radiator can have lower losses, leaving more of the available tx power for the radiator, which increases the field it can generate. This, and not the “total surface area” of the radiator is the reason that an antenna system with a larger diameter radiator has slightly better system efficiency.
The more constant capacitance of the larger diameter radiator over the operating bandwidth means that the SWR at the feedpoint stays lower across those frequencies, which in turn allows better power transfer of program modulation into the radiator.
But the biggest and least understood factor affecting the radiation efficiency of the complete antenna system is the r-f resistance of the ground connection. This is not just the DC resistance of the conductor(s) leading from the tx chassis to something buried in the earth (ground rods, etc). It is the resistance in the “ground system” to the r-f currents induced into the earth by radiation from the antenna.
These r-f currents need to be returned to the tx in order to complete the path needed for that current to flow in the radiator. If most of them can’t be collected from the earth, the antenna won’t radiate well.
The r-f currents exist in the earth out to a distance of about 1/2 wavelength from the vertical radiator. Collecting them efficiently requires providing a metallic path for them to return to the antenna system. Broadcast stations typically bury 120 radials of at least 1/4-wavelength each. Such a radial system has a resistance of about 2 ohms to the returning earth currents.
The r-f resistance in the buried ground system path of a typical Part 15 AM antenna system will be considerably higher than 2 ohms.
Here’s how the radiation efficiency of a Part 15 AM antenna system is calculated, using ~typical values.
A: Radiation resistance of 3-meter vertical
at high end of the band = 0.1 ohmB: Loading coil r-f resistance = 2 ohms
C: Resistance in the ground system (r-f resistance) = 20 ohms
Radiation efficiency of the antenna system = A/(A+B+C) = 0.1/22.1 = 0.45%
So this antenna system will radiate 0.45% of all the power that the tx can supply to its feedpoint. The rest is lost as heat.
The biggest defining element by far in the efficiency of a ground-mounted Part 15 AM antenna system is the loss in its ground system.
//
Total posts : 45366Rich,
Could you possibly mean .45 X 100 = 45%? Otherwise, the typical Part 15 antenna system that can reach a mile or more is radiating less than 1/2 percent of the 50-60 percent (efficiency of the PA) of the 100 milliwatt input to the final amplifier. That would mean something like nano-amps of current at the feedpoint of the antenna. That would indicate (Ohms Law) a very high impedance at the feedpoint, which you correctly explained is not true.
Broadcast AM stations determine the efficiency of antenna systems by 2 methods. One is by field strength at one mile. The second is the amount of transmitter output necessary to produce a specific base current that produces a specific mv/m field strength. No computers or software needed to determine the efficiency. Pencil and paper work fine. I have built and consulted several AM stations with signal problems using the same method. And when using the antenna bridge to measure the base impedance, amazingly the results are within statistical and mathematical factors of acceptable error. RF antenna design is not a science, it is an art. Build one of the bog multi-stick arrays and you will see why.
And by the way, increased surface area or diameter does decrease the impedance of the antenna, allowing more current to flow in the conductor. Additionally, the bandwidth of the antenna system is improved making the program audio in the sidebands louder and, ultimately, the signal louder and more listenable. Increasing the power in the sidebands of AM has been the life’s work of Leonard Kahn. You might want to read some of his white papers on the research of the “Powerside” system.
The largest challenge Part 15 stations face is the lack of radiation resistance in the super short antenna. Even with a greatly enhanced Part 15 ground radial system, the increase in signal strength is minimal in decibels.
Thank you for your helping to explain the theory necessary to get better at understanding what we do.
Marshall Johnson, Sr.
Rhema Radio – The Word In Worship
http://www.rhemaradio.org
Total posts : 45366[quote=kk7cw]Could you possibly mean .45 X 100 = 45%? [/quote]
The equation to calculate antenna system radiation efficiency is simply Ohm’s law for a series circuit containing several resistance elements. The tx supplies some value of r-f current to it, which has to flow through all of those resistances. Part of the total power of the tx is dissipated in each of those resistances, in accordance with the ratio of each resistance to the total resistance. The power consumed by the antenna in my example really is 0.004525 times the total power applied, which I expressed as a percentage (~0.45%).
[quote=kk7cw]Otherwise, the typical Part 15 antenna system that can reach a mile or more is radiating less than 1/2 percent of the 50-60 percent (efficiency of the PA) of the 100 milliwatt input to the final amplifier. … That would indicate (Ohms Law) a very high impedance at the feedpoint, which you correctly explained is not true.[/quote]
The impedance at the base of a 3 meter Part 15 AM antenna IS extremely high. It consists of the radiation resistance and reactance of that radiator on that frequency. The radiation resistance will be very small (~0.1 ohm or less) and the reactance will be very high (several thousand ohms). Only the radiation resistance can dissipate power (which radiates), but no practical tx can deliver much power into such a highly reactive load. That is the reason for the loading coil — to cancel the high capacitive reactance of the short radiator so that the tx has a usable load for its output power.With the coil, the feedpoint impedance at the coil input contains the original radiation resistance plus the added r-f loss in the loading coil, but with ~ zero reactance. This is a load impedance that the tx can deliver power into. Now we revert to my comments just above to see how much of that total power of the tx actually is radiated — ie, to determine the radiation efficiency of that antenna system.
[quote=kk7cw]Broadcast AM stations determine the efficiency of antenna systems by 2 methods. … No computers or software needed to determine the efficiency. Pencil and paper work fine. I have built and consulted several AM stations (etc) [/quote]
Pencil and paper also work fine using the method I posted above. And our methods are the same, anyway.The impedance measured at the base of a tower of a licensed AM broadcast station includes all the parameters I have described in this thread: radiation resistance and reactance, matching network resistance and reactance, and r-f ground loss.
The methods you described for determining AM broadcast station performance would work equally well for Part 15. It’s just that test equipment to do that is expensive, and probably not justifiable for most hobbiests. And likewise, the method I described also would work equally well for licensed AM broadcast stations if the impedance values of the various elements of the antenna system are separately known.
[quote=kk7cw]And by the way, increased surface area or diameter does decrease the impedance of the antenna, allowing more current to flow in the conductor.[/quote]A conductor with a large OD is no more efficient a radiator by itself than one with a much smaller OD, when both are resonant, and have the same current flowing in them. I think this point is not being fully recognized by your statement, although I expect that you understand this yourself.Anything that increases the amount of current flowing in any radiator will increase the fields it produces. One way of doing that is to improve the impedance match between the tx and the antenna feedpoint. But in the case of a larger OD radiator, the increased radiation field is not related to the larger surface area in a direct way — it is a secondary effect.
For calibration, here are calculated values for two 3-meter conductors of two ODs, on 1,700 kHz:
Radiation resistance > Reactance > VSWR referenced to 50 ohms
1/2″ OD: 0.12 ohms > -2,498 ohms > over 1,000,000:1
4″ OD: 0.12 ohms > -1,335 ohms > about 300,000:1So in neither case will a tx be able to supply much power into such mismatches. A loading coil will be necessary, but the one for the 4″ OD radiator probably will have less loss than the one for the 1/2″ OD radiator, and that can give some relative improvement in radiated field.
[quote=kk7cw]Additionally, the bandwidth of the antenna system is improved making the program audio in the sidebands louder and, ultimately, the signal louder and more listenable.[/quote]The bandwidth of the tx is set by its modulating characteristics. How much of that modulation gets radiated is set by the bandwidth of the antenna system. It won’t matter if the tx can be modulated at 15 kHz if the match of the antenna system at those sideband frequencies rejects them.
[quote=kk7cw]The largest challenge Part 15 stations face is the lack of radiation resistance in the super short antenna. Even with a greatly enhanced Part 15 ground radial system, the increase in signal strength is minimal in decibels.[/quote]
Yes, the low radiation resistance is a big contributor to the coverage performance of Part 15 AM. But the effect of the r-f ground is not really minimal, either.The r-f ground resistance of a Part 15 AM antenna system using a “broadcast-type” radial ground system might be on the order of 2 ohms. If the buried ground system consists only of a few ground rods at the base of the 3-meter radiator, that r-f ground resistance might be ~ 50 ohms (or more).
Using the method from my earlier post about a 3-meter Part 15 radiator, here’s how those numbers crunch when determining radiation efficiency with those two ground systems.
Broadcast radial ground system ( 2 ohm ground resistance):
0.1/4.1 = ~0.0244 = ~2.44%A few ground rods (50 ohm ground resistance):
0.1/52.1 = ~0.00192 = ~0.192 %The power ratio of 0.0244 and 0.00192 is about 11 decibels, which is very significant — not minimal.
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Total posts : 45366A transmitting antenna is essentially a passive circuit consisting of resistance, inductance and capacitance. Some of the load components of a radiating element are virtual in nature and cannot be measured. Radiation resistance is one of those virtual load components. We calculate the radiation resistance with Ohm’s Law. Much like the classic “tank circuit”, a transmitting antenna needs to possess the “flywheel” action in order to resonate. A resonating antenna doesn’t mean it’s tuned to 50 Ohms resistive. I just means the antenna is very near minimum resistance and zero reactance.
With all of this said, if an antenna system is designed with minimized ground resistance and maximized radiation resistance, then the efficiency of the system should increase. Graduates of the U.S. Naval Academy decades ago did graduate research of the effect of a folded vertical element above an efficient ground system and also the effects of a less than perfect ground system or counterpoise. Basically, a folded dipole is accepted as having a charactersitic impedance of 200 to 450 Ohms depending on the antenna conductor and fold conductor sizes and the spacing between the two. The study centered on the use of half of a folded dipole or what is called a folded monopole or unipole. These antennas have been used for years to improve the radiation resistance of conventional AM uniform section iron tower antenna systems. The concept is also fully discussed in the ARRL Antenna Handbook and the Radio Handbook by Bill Orr, W6SAI.
And as a final antenna consideration, I suggest a look at the Isotron Antenna Company. The Isotron Antenna is nearly a pure reactive tank circuit. As transmitting antenna, they do work well. And for the past couple of years have manaufactured a model for Part 15 broadcasting. I wonder how your SSTran unit would work hooked up to one these babies? Not a very conventional antenna design, but might be worth the look.
http://www.isotronantennas.com/ambroad1.htm
Marshall Johnson, Sr.
Rhema Radio – The Word In Worship
http://www.rhemaradio.org
Total posts : 45366[quote=kk7cw]Graduates of the U.S. Naval Academy decades ago did graduate research of the effect of a folded vertical element above an efficient ground system and also the effects of a less than perfect ground system or counterpoise. …. The study centered on the use of half of a folded dipole or what is called a folded monopole or unipole. These antennas have been used for years to improve the radiation resistance of conventional AM uniform section iron tower antenna systems.[/quote]
There are two issues to mention here, though.1) The real benefit of the folded unipole for those who can use them is that they allow the use of a grounded tower, which eliminates the base insulator. It is just a method of shunt feeding that tower. For best results the tower needs to be at least 1/4-wave tall, which means they are not real useful for Part 15 AM stations.
2) The input impedance to the folded unipole can be somewhat arbitrary, and even adjustable to directly match a 50 ohm transmission line connected to its feedpoint. However the field radiated by the folded unipole is no greater than supported by the power rating of the transmitter (less r-f ground losses) — and will be the same as if using a conventional, properly-matched, series-fed monopole of the same height. So nothing is gained, as far as improving station coverage.
The fact that the fields from these two are the same for a given applied tx power is implied by the fact that the peak gain of a 1/2-wave dipole in free space is the same as that from a folded 1/2-wave dipole in free space.
For more on this, please read Section 2.3.4 of “Radio Antenna Engineering” by Edmund Laport (McGraw-Hill, 1952).
[quote=kk7cw]And as a final antenna consideration, I suggest a look at the Isotron Antenna Company. The Isotron Antenna is nearly a pure reactive tank circuit. As transmitting antenna, they do work well.[/quote]Note the following paste from the Isotron URL you posted:
Antenna should be mounted on a metal mast with a maximum size of 1.5 inches. The mast can be ground-mounted with guy wires, mounted to a wooden pole, or mounted to a tower leg.
Antenna should be mounted as high as possible for best performance.
The Isotron includes some top and bottom loading plates on the vertical radiator section. Those will reduce its input reactance by some amount, but won’t much change the radiation resistance of that vertical section (which will be very small). A loading coil cancels the capacitive reactance that is left.
Now note in Isotron’s clip above that it needs to be connected to a metal mast, and installed as high as possible. The mast itself would be connected to some sort of r-f ground, so the height of the grounded mast is added to the radiating length of the Isotron.
This is the same operating scenario as with the “elevated Part 15 systems” that I have been posting here, and written a short paper to evaluate (copy available on email request to rfry@adams.net).
IMO some of the beliefs about such antennas are based on an interpretation of marketing statements, more than anything of revolutionary technical merit.
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Total posts : 45366Hello Rich,
I just have one question for you: Is there a way to reduce loading coil losses? I read on some websites that i must to make a loading coil with a bigger diameter form. And some others said that an air core coil is the best. There have one store near from my home that they sell some air core coils. For example, 0.6 mH air core coil costs about 4.99$. I know the tricks about cutting somewhere in the air coil for bringing back to 0.35 mH.
I would like to have your opinion.
Regards
Yves
Total posts : 45366I haven’t given the specifics of this a lot of thought at this point. I’ll see what I can find in the literature. In general, I expect that coil dimensions giving the needed reactance and comprised of shorter total lengths of larger gauge wire probably will be better. I think you know about the DOS programs of Reg Edwards, and I believe he has several dealing with this subject.
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