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[quote=kk7cw]Once again, the challenge any short antenna system has is efficiency. The Isotron antenna's approach to improved efficiency is to use lower loss components. And since the model 200B does need a radio frequency ground system to work, it is legal under Part 15 for AM........

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
[/quote]

Has anyone out here used one. I know of some use on amateur radio but much higher in frequency and as one would suspect, the BW is quite sharp. (narrow)

You might want to check out this thread about a guy built his own Isotron antenna. Although it was tricky to tune, he was pleased with the results. There are pictures of the finished antenna and he explains how he built it.

As you know, there aren't many MW antenna designs that eliminate the need for ground radials. The EH antenna is the only other one that comes to mind and, so far, the published results have been inconclusive.

http://www.pcs-electronics.com/phpBB2/viewtopic.php?t=1092

DB52

Is there away to couple this to the AMT 1000 without a boat load of mods?

WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager

[quote=Rich]
Sorry, but you might want to research this a bit further. A buried r-f ground consisting of radials, ground rods or other conductors, and used with a vertical "monopole" MW radiator only serves as a return path to the tx system for r-f currents induced in the earth by the MW radiator. Not supplying a return path for those currents is similar to connecting something to only one terminal of a battery -- current doesn't flow. And in the case of a Part 15 AM antenna, no/very low r-f current flowing in the antenna system means zero/very poor radiation efficiency.
[/quote]

Ok, I'm probably jumping in way over my head here, Rich.. But looking at the schems of the 80 meter version of the antenna in the manual (page 19 of the pdf) at:

http://www.isotronantennas.com/isomn80.pdf

..ok, now that's 3 of them linked together, but it was a place that shows a simple schem of the beastie.. Now it *looks* like electrically it amounts to a sort of physically large tuned tank, somewhat like some of the old loop antennas, though with the parts set up in maybe a different physical arrangement.

Now, with a straight "stick" vertical being fed from a tap on the loading coil and a tuning cap between the base of the coil and the ground, we'd clearly have an "open circuit" if it were not for some of the radiated power returning to the antenna's ground via something like buried radials.

But looking at the Isotron antenna schematics, it looks like a simple series LC "tank", so wouldn't that circuit provide the path for current flow without the ground radial system needed for a standard vertical? I definitely agree that the standard vertical needs the ground return to "make a circuit".. But this looks a bit different from that to me, at least at first glance.

Now the real questions I'd have about the Isotron antenna...

Since a series LC offers zero impedance at resonance, wouldn't a transmitter see the "load" from this thing as basically a short circuit? Like connecting the antenna terminal right to a ground? I suppose the resistance value from the coil winds might prevent that, but it was one of the first thoughts I had.

Also, what affect is the close proximity of the earth going to have in an antenna where the top of it is only 3m off the ground? I could see where this design might be theoretically superb if one was transmitting from say a hot air ballon with it hanging underneath, where one could easilly be a wavelength or more off the ground. But I'd think that in any practical application where it's mounted a few feet or meters above the ground, the earth would act like a capacitor plate, so height could drastically affect tuning?

Also, those capacitor plates are rather large. Could wind on them result in enough vibration to add unintentional modulation sort of like a rather odd and large condenser mic? Or maybe even any loud nearby sound like a truck passing.

As wdcx already mentioned, one would expect the tuning to be a bit touchy and the bandwidth to be pretty narrow. Might be great for cw or something like a 200 hz test tone for a beacon, but would the bandwidth even be usuable for music at a reasonable fidelity?

Sorry if some of those questions are a bit nave, but anybody have any thoughts on any of them?

Daniel

[quote=Rich]
IMO radiation from a Part 15 Isotron wouldn't be much different than from a normal, base loaded, short radiator.

[/quote]

Ok, so if I understand you correctly, you're saying basically it still looks a lot like a base loaded vertical (albeit with a sort of unusual capacitance "hat") because that's pretty much what it is. And as such the company's claim that:

[quote]
The ISOTRONS exceed or equal (depending upon the model) the area of a conventional one-half wavelength dipole (#12 wire)...

...Tests have measured the ISOTRONS to transmit as well as a one-half wavelength dipole.
[/quote]

...is probably carefully phrased ad hype. So assuming both are mounted on or very near the ground, it wouldn't do much differently than the base loaded verticals usually discussed here?

Ok! That's what I was wondering when I looked at the design.

Thanks, Rich.

Daniel

Having started this thread I needs say when talking to the manufacturer while ordering my Isotron and he told me that the ground on the outside mounted Isotron was for discharging static electricity. I believe somewhere on their site they state that an indoor or apartment patio mount doesn't require a ground. I plan to use the antenna with a PCS electronics AM Max II transmitter although I'd like to try it with my SSTRAN too. It's on the PCS site Forum that the home made Isotron is discussed along with Isotron entries on several of their Forum threads from which one can piece together the necessary information for a home brew. As to part 15 compliance of the Model 200B, although the height is less than 3 meters, when the additional length of the capacitance caps are added the 3 meter length is exceeded. I do believe the FCC counts the capacitance cap length. JimB

Do you mean to use a Rangemaster AM1000 with an Isotron? You bethca. Just insert a 3/8-24 X 1/2 stainless bolt, with lock washer, in where the whip normally connects and attach a "short" wire between the transmitter and the antenna feed point. Connect the transmitter ground as directed by the xmtr manufacturer (ground side of the antenna) and you should be in business. Remember, you have only 3 meters to play with for length. Also, you will need to carefully re-tune the transmitter to the new antenna. Be careful of "false" peaks in the tuning indication.

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

[quote=kk7cw]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.[/quote]

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.

[quote]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.[/quote]

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.
//

[quote=kk7cw]...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.[/quote]

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.

[quote]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.[/quote]

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...?
//

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

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

[quote=jbprptco]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[/quote]

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

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

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...

Mr.Marshall Johnson,
Thank you for your response/attention .fortunately here in Indonesia ,we may build licensed Medium wave broadcast radio with power from 500 watt to 1600 watt (transmitter I have build) ,and we used it as Gospel Radios.
The problem we have, is building an Efficient antenna,WE don't have enough land and fund to build a full quarter wave antenna ,and especially laying the quarter Rf ground radial.
So after looking in the Internet trying to find a solution, I have read about EH antenna,Gap Antenna,CFA antenna, Isotron antenna, A small medium wave antenna ,and according the designer it's a good Antenna.
After searching the Internet what a coincidence,I've found Your web discussing/using the Isotron Antenna.
May be you can help me,and give me advise building a small compact Medium Wave antenna that's an efficient Antenna.

Thanks

Gito.N