Well stated. I remember while attending an EMP/ESD seminar the instructor stated, "Ground is not ground the earth round..." LOL!!
The Isotron is, by design, a tuned L-C tank circuit, resonating within itself. The characteristic impedance of the device over ground produces the propagated radio signal.
From their appearances, the Isotrons for "ham" radio applications are constructed differently than the one for Part 15 AM. The ham versions appear to be very short, loaded doublets (dipoles), which are balanced antennas not needing a connection to an r-f ground to radiate as efficiently as that design allows. Not so for the Part 15 version.
Also, just a note: I have engineered and constructed several licensed AM broadcast "Unipole" or folded monopole antennas that seem to work just fine over a non-existent or greatly diminished ground system. Theoretically, it shouldn't, but in practice the measured field strength is significantly greater than a conventional 1/4 wave tower with NO change in the ground system itself. No real explanation why.
I guess this depends on one's idea of what "significantly" means. However, this observation hasn't been demonstrated by a careful comparison of the measured fields produced by these two types of antennas with different qualities of ground systems. The consulting firm of duTreil, Lundin and Rackley made such comparisons, and reported on them in a paper they presented at the NAB Engineering Conference in 1996.
Here is a quote from the conclusions section of that paper. "No major differences in field strength between the folded unipole and series-fed test cases were found for any of the configurations tested. The folded unipole was not found to have significantly better radiation efficiency than the series-fed for a given tower height and ground system." The complete paper is available at http://www.dlr.com/pdfs/ComparisonWaveFedVsSkirtFed.pdf
A v-pol MW folded unipole really is just half of a folded dipole, and so for the same reason as for a standard monopole, a very good ground system is important to it -- in order to return the radiated displacement currents from the nearby earth back into the tx system.
//
Rich,
Once again I have read and agree with the outcomes of the 1996 study, but also with the original study done at the Engineering Dept. of the U.S. Naval Academy is the 1970's. All of the studies used "some" computer modeling and the NEC code to verify their results. I do not question their testing methods or results.
About 8 or 9 years ago, I had an opportunity to speak with (via telephone) the inventor of the original NEC code. Unfortunately, I don't remember his name. He was quite frank about the use of of "his" NEC coding for modeling radio systems. He pointed out on several occasions, that using the NEC codes for radio system modeling gives the engineer a "model" of what should happen in the real world. But because of environmental and other anomolies, the results are only a "model" and not imperical evidence of the result.
The reason for my questions to him were the result of my questions of "significant" differences in system comparisons and results (the original vs the unipole or other). What I had designed (modeled) and what the built systems actually produced in the real world applications were different. Additionally, an antenna company called Force-10, had directed me to this person to get first hand information on antenna modeling software based on NEC modeling. And, obviously, I got an earful.
I have actually designed and built several folded unipoles with very similar results in every case: a significant measured increase in field strength (using a calibrated field strength meter indicating +3-4 db) at one, five and ten miles from the system over the original system. I have asked people, such as yourself, for years, why. Just as you have explained, the data shows it shouldn't perform any differently than the original system, but the truth of the matter is; it does. Field strength is field strength, millivolts are millivolts, microvolts are microvolts, decibels are decibels. An increase is an increase. I have even returned to the installations a year or two after the installation and made more measurements; the results were the same. And as such, the FCC has agreed with and licensed the results. So, it must not be physical science. It must be divine intervention or something.
As to the Isotron antennas, I suggest you do more research into their design and construction. You may have missed something there. Also, talk to some folks who use the system and dial their experience and data into your findings. I will be very interested if you find what I have found.
Rev. Marshall Johnson, Sr.
Senior Pastor, President
Rhema Christian Fellowship, Inc.
Rhema Radio - The Word In Worship
AM 1660 - FM 93.5
http://www.rhemaradio.org
...The reason for my questions to him were the result of my questions of "significant" differences in system comparisons and results (the original vs the unipole or other). What I had designed (modeled) and what the built systems actually produced in the real world applications were different.
NEC has to be used with due understanding in setting up an accurate model, and when that is observed it will give quite accurate real-world results. Even NEC-2 can be used to model a vertically-polarized radiator connected to a perfect ground through a few ohms of resistance to simulate typical ground loss in the radials. This produces the intrinsic patterns and gains of that radiator, which leads to its FCC "efficiency." That value used with the FCC's MW propagation charts for the frequency and path conductivity are quite accurate in predicting daytime groundwave field strength for the given conditions.
A rough example of this is shown in paper 5 at http://rfry.org/Software%20&%20Misc%20Papers.htm . The NEC numbers there don't exactly match the FCC numbers there, but they aren't too far off in terms of decibels.
Broadcast consultants routinely use NEC-4 to develop the patterns/gains of MW directional arrays, which when built and measured must and do meet the limits of the FCC construction permit.
I have actually designed and built several folded unipoles with very similar results in every case: a significant measured increase in field strength (using a calibrated field strength meter indicating +3-4 db) at one, five and ten miles from the system over the original system.
At first you seem to agree with the conclusions in the 1996 duTreil, Lundin and Rackley paper showing no significant difference between the groundwave field strengths of folded unipoles and standard series-fed monopoles with the same height and ground systems. But then you say you have measured such fields and found them 3-4 dB different in favor of the folded unipole. Something must account for your observations, but lacking specific knowledge of the conditions it is impossible to pursue that here. I do know that the firm of duTreil, Lundin and Rackley is one of the top consulting firms in the broadcast industry, and their work has been accepted as authoritative for many years.
This exchange probably is getting well beyond the general interest of Part15.us readers, so maybe we should take it off line if there is more to discuss...?
//
I asked Keith about doing the same thing, and he said it wouldn't work because the AM1000 expects a high impedance antenna, and the Isotron has a low 50ohm impedance. The reason I asked is that we have RG8 running through our building and up a tower that would have worked perfectly betwen those two devices if it wasn't for the impedance mis-match.
Don't know if he really thinks it is a bad idea, or if it's nonsense. He also said that the part 15 rules might expect that the transmitter gets used with the antenna that hey used to certify the transmitter. Again, I don't know if that is right or wrong since I didn't check into it. And I can't seem to get anyone here interested in an AM device that will at least reach off campus.
Greg,, I haven't seen you on any previous threads so don't know don't know what questions you've asked about campus broadcasting, yet let me suggest carrier current broadcasting which is used on many campuses. Go to the search engine on the left hand column of this page. Make sure the part15.us dot is highlighted and enter into the search engine "carrier current." you will find a lot of information from our part15.us library. As to Keith Hamilton and Phil Bolyn of Rangemaster and SSTRAN respectively, I believe them to be extremely compitent electronic engineers and that they have developed their respective transmitters/antennas to perfection as they are already presented. Not much can be done to improve them. Remember, a 100mw signal isn't much power to work with. With carrier current you may use much more power on campus. You're limited as to the distance the signal may radiate past the campus boundries. All of that information will be given in the library. JimB
Remember, you are not bound to the 100mW rule on the college campus. There are special rules for AM broadcasting so long as the signal is confined the campus.
A reality check.
The Isotron is a base loaded vertical in disguise. The "vertical" part is not a whip or copper pipe, it's a bunch of odd shaped plates of sheet metal. This serves to increase the capacitance of the vertical element, which slightly reduces the number of turns required in the loading coil (included in the antenna). The number of turns in the loading coil isn't important practically from a performance standpoint. Radiation resistance may be reduced somewhat (a good thing) by these plates, but the reduction is swamped out completely by the typically high ground loss resistance of a practical part 15 ground setup.
I don't want to disparage anybody who is honestly selling a good product, but this one doesn't add up to the advertisements technically. Take a close look. If this is such a great product, why don't you see at least one post from someone who reported increased range with all of the myriad of other variables kept constant.
Phil B
In case any of our forum members aren't aware, PhilB is the designer and manufacturer of the SSTRAN AMT3000 and the associated base loaded verticle antenna. Phil, 2 questions: is there any place in the circuit of the base loaded verticle where a variable capacitor may be placed in place of the sliding copper pipe adjustment, still giving the same efficiency? And, would you kindly comment on the use of a copper pipe as the verticle radiating element. On recent threads here some experimenters express their interest in a CB whip. I understand the thicker copper pipe widens the bandwidth of the signal, hence the audio frequency response or sound quality. Is that true? Thanks Phil and good to hear from you again. JimB
2 questions: is there any place in the circuit of the base loaded verticle where a variable capacitor may be placed in place of the sliding copper pipe adjustment, still giving the same efficiency? And, would you kindly comment on the use of a copper pipe as the verticle radiating element. On recent threads here some experimenters express their interest in a CB whip. I understand the thicker copper pipe widens the bandwidth of the signal, hence the audio frequency response or sound quality. Is that true? JimB
Hi Jim,
Yes, a variable cap can be used in place of antenna length adjustment, but from my experience is not quite as efficient. A large (720pf) variable can be placed in series between the TX and the coil. The large value is required to get decent adjustment range since the variable is essentially in series with the ~30pf antenna capacitance. A large variation of the variable has a much smaller effect on the series capacitance. You can't skimp on the quality of this cap, so it will be expensive. Cap loss is added to the overall losses. That's why it's not quite as efficient. Its better to put the cap on the low-impedance feed side of the coil where it's not part of the antenna and is less sensitive to hand capacitance.
A small variable cap on the order of 50pf can also be connected to TX ground on the antenna side of the coil. The peak voltage here can be up to 250volts so the cap must be appropriately rated. Once again, cap losses are a factor. A cheap small trimmer will have higher losses and may not handle the voltage. The SSTRAN antenna uses length adjustment for varying the capacitance. It's a pain from a construction standpoint, but is the most efficient.
A CB whip can be used. The fixed length of 102" forces the overall length of the antenna to be somewhat less than 3 meters Compared to the copper pipe, the effect is probably negligible. I have searched several times for off-the-shelf hardware that could be used to construct a simple and sturdy length adjustment mechanism for a CB whip. I have not been successful in my search. Custom hardware could be fabricated, but would by relatively costly. If anyone has any ideas, let me know. SSTRAN does not sell the antenna. The design is in the public domain. Any suggestions for improvementa are welcome!
Base-loaded antennas, by their nature, are very narrow-band, essentially regardless of the OD of a legal radiator. The Q-factor is very high which causes a sharp tuning peak. A lower Q-factor would broaden the peak, but at the expense of more loss. The high Q is a good thing for reducing losses and reducing harmonic radiation. The narrowness of the peak does have an effect on audio BW, but in practice it isn't all that bad and can be compensated with audio high-freq preemphasis. A good transmitter can still exceed the high freq response of virtually all radios, even with the sharply tuned antenna.
Phil
Since Im finally getting people thinking that AM might be our best option, I have a question relating to this antenna thread. How does the diameter of the pipe affect the sound quality of the signal. Obviously there is a certain trade-off somewhere, but does a 1 inch (or larger) diameter pipe make a noticeable difference over a 1/2 inch pipe?
Has anyone seen any difference is performance by using a different material (like stainless) instead of the copper.
One mod I might suggest, and one that I'll try if we end up building an AM system, is to use a pipe coupler instead of cuting the 3/4 inch pipe. 3/4 to 1/2 reducers should be very easy to find, and all that would need to be done is to grind out the stop so that the 1/2 inch pipe can be pushed all the way through the coupler. Once you get your tuning set, you could use solder to fix the antenna. This would let you fight off corrosion at the joint, and keep water from running down inside the antenna. It should also be stronger for high wind use. The same could be applied to larger diameter pipes.
If you didn't want to solder the joint, you could probably use some sort of clamp to fix the two pieces together, though it still won't slide well after the metal is distorted.
Some clarification to my last post is in order.
The Isotron has some flat plate elements above and below the base of the antenna loading coil. This configuration can be compared to "top hat" base loaded vertical antennas.
A standard base-loaded antenna has a linear antenna current that is maximum at the bottom and zero at the top. A "top hat" antenna also has a linear current that is maximum at the bottom, but slopes more gradually to the top where it abruptly goes to zero. This increases the radiation from the 3-meter antenna. The Isotron probably has a somewhat better (but unknown) radiation level than a simple base loaded vertical, but I don't think it could measure up to a real top-hat vertical.
Even in the ideal case that the Isotron would be able to achieve an antenna current profile that is constant from bottom to top, it would still not make a significant difference because, as we all know by now, an elevated 3 M antenna radiates mostly from the ground run, so minor differences along the little 3 M section at the top are insignificant.
Phil B
Dear Marshall Johnson.Sr.
I'm a new comer here. I lived in Indonesia I made small M.W transmitter,and help building many Gospel Radio,in Papua/Irian ,Sulawesi,Halmahera,and planing to build in Kupang /West Timor,The problems we have,is Building an efficient short profile M.W antenna,because we don't have the land and fund ,to build a full size quarter wave antenna with ground radials,so the efficiency of our Antenna is poor.
I've heard off many new short profile Antenna like EH antenna,Gap antenna,CFA antenna,but have not enough information to build Them.
So when I read article about This Isotron antenna,I'm very interested.
Marshall can You help by giving detailed information and advise how to build this Isotron Antenna,how to calculate the lenght ,the coil, how to tune this Antenna.
or you can advise to build another short profile M.W antenna that's efficient enough .
The antennas we have build is a Short top loaded capacitance
hat,with ,bottom loading/tuning coil without ground radials so I think the Efficiency is poor.
Thanks
Gito.n
Gito,
Remember, the impedance of the antenna should be as close as possible to the output impedance of the transmitter. Many "part 15" transmitters are designed with a high impedance output. And,because of the regulations in the U.S., our MW antennas can only be 3 meters or less in length. That is the reason for the loading coil; to bring the impedance of the short antenna with in a reasonable range of the transmitter impedance.
Your brief description of the antenna systems you are using sounds like you're on the right track. However, you must add ground radials under the antenna to increase the radiation efficiency of the antenna system. Theoretically, think of the radial system as the other half of a dipole antenna system. 16 or more ground radials (20-25 feat or longer) will produce a remarkably louder signal. Usually 4 elevated radials of the same length will produce about the same results.
In your country, depending on the regulations and the transmitter's design, you might try a simple dipole within 5-8 feet of the ground. This would produce a decent ground wave plus a near vertical incident skywave or NVIS type of antenna. NVIS antennas are excellent antennas for short distance communications. Check the internet browsers for more info on how to build this antenna system. I have built several, and they work really well for what they are designed for. By the way, military units worldwide use NVIS antennas for tactical short range communications on HF.
Gods Blessing to your ministry...
if anyone gets a isotron for part 15 am, please draw out a schematic with dimensions. i have a 40 meter or 80 meter isotron that i would not mind canablizing for a part 15 am project 😉
i may even build one from scratch then tweak the design a little to see what happens.