Using a CB Antenna for top 4 feet of a 3 meter long AM antenna

From what I have read and seen with the recent KENC fiasco...as well as consulting with my contact at the Denver Co. FCC field office, if you connect the TX ground to the mast and have no RF choke on that ground lead, that mast will be considered as part of the total length of your antenna system which will include the radiating element. And that is with the mast no taller than 3 feet off the ground.

My contact advised me that with any antenna arrangement for a Part 15 installation, always install the RF choke on the ground lead BEFORE it exits the transmitter box and connect to the mast. This will minimize any RF emitted off the ground and mast.

A typical 100 microhenry choke from radio shack will do the job, but a better choke is by making one in a toroid form and using #16 AWG or slightly larger solid conductor wire with about 10 to 15 turns to maintain ground conductivity while attenuating any stray RF from passing on to the ground connection or mast.

Now if you install the TX right at the surface of the ground, you wont need any RF choke at that point. In fact I have found such installations to work far better than to be hoisting the whole works up on some tall mast or billboard sign, simply because the ground return path is shorter with the TX at ground level than it is to have it hoisted up on a mast and using the mast as your ground path even with a choke.

My backup intentional radiator system sits at ground level with a 1 foot 2 inch pipe as a mast. The TX box mounts flush with the ground floor and the ground lead is 2 inches connecting directly to a ground rod with 10 buried ground radials extending outward in a circular pattern. A choke is added inside the TX box to the 2 inch ground lead to positively block any potential RF to the ground path.

The radiating element is exactly 9 feet 8 inches in length and sports a home brew loading coil inside the TX box with a remote controlled servo connected to an old AM radio variable capacitor to fine tune the entire system remotely from inside. A sensing loop consisting of 1 turn of #22 AWG wire around the base of the radiating element inside a rubber washer and feeds an op amp to send a DC voltage proportional to the emitted RF signal for metering on my antenna tuning control panel. I also have this panel equipped with final current metering and B+ metering. These meter indicators are duplicated inside the TX box outside and I regularly calibrate the system once every two weeks to ensure compliance for 100mW final input and maximum resonance of the antenna system. It serves well for a backup system to my carrier current station.

RFB

Thanks RFB. That is great info. I like your RF monitor idea. The choke on the ground wire makes sense, and I gather it's a get out of jail free, pseudo approval to not count the ground wire. That's cool. I gather Part 15 means no complaint no foul and no gratuitous violation of the "spirit" of the reg.

My transmitter is a Talking House that has the audio mod and sounds great. Right now it sits near 2nd floor window in my house. The window frame is vinyl with a non metallic screen. The 9' wire hangs out the second floor window against the vinyl siding. The Talking House has a built-in antenna tuner.

With the price of copper and the fact it's working well with the indoor installation, my jets are cooled on making this antenna, but I think this spring I'll make it. I might look for a Talking house ATU, so I can use the coaxial feed. I enjoy learning, and trying to make a good antenna sounds fun. I got sticker shock shopping for copper wire and pipe. Ouch!

Let me ask you with all this effort to make a good antenna, good ground system, most part 15 AM transmitters must exceed the field strength limits, which is tiny. The field strength at 30 meters on 1380 Khz is 24000/1380 = 17.39 uV/m, which is really low, I think below most radios sensitivity and noise floor. Even with my poor system (?) my good SW portable can get the station at 30 meters. At about 42 meters I get some loss of signal. A good outside antenna and ground system would only improve this (or make it worse depending on your perspective). I gather if no one complains than all is good. Regardless you want to have a legal system. The advantage of a good outside antenna in the back yard is better sound (through a stronger signal). Since I don't have an accurate RF meter, I'm blissfully ignorant of my transmitters true field strength.

For now I get my whole smallish 1800 sq ft wood frame two story house, plus 140 feet outside. Nothing to write home about but works well for my needs, playing music, internet radio or pod-casts on my restored tube radios.

PS what do you think about incorporating a CB 4' whip to the top of my antenna plans? I guess the only way to go is make it experiment.

Let me ask you with all this effort to make a good antenna, good ground system, most part 15 AM transmitters must exceed the field strength limits, which is tiny.

Well if we take the rule book to the letter, the actual limiting factor as described in the rules is the 100mW maximum input to the final RF stage. (15.219).

The well built and tuned antenna system will simply let that little signal transfer at its best. This may or may not affect a field strength reading at the given distance due to surroundings and other factors. Part 15 section 219 does not specify a field strength limit, only that the maximum input power must not exceed the 100mW level, and antenna with lead and ground must not exceed 3 meters. However in parts 221 and 209 DO specify field strength limits at a given distance from the radiating element. Part 221 are for carrier current and leaky coax systems.

This is why it is vitally important to operate under the specific part for your installation. 219 is always referenced for intentional radiators, ie antenna..be it a wire, copper rod, coiled fiberglass whippy or even a fence.

Lets put it this way..if there ever is an intentional radiator design that is 100 percent efficient and throws that signal 2 miles while adhering to the 100mW maximum RF input to the final RF stage, that installation is LEGAL period. However no one has yet to design and build an antenna system, even for the licensed stations that is 100 percent efficient. I do not think that will happen anytime soon unless there is some black op project taking place somewhere using materials alien to this world that yields 100 percent efficiency. 😉

PS what do you think about incorporating a CB 4' whip to the top of my antenna plans? I guess the only way to go is make it experiment.

Well it sounds like a double loaded antenna design from what you are describing. However here is the catcher. Any loading coil that sits anywhere on the radiating element, be it at the bottom, top, middle or combination of more than one...IS radiating RF energy. This is why some installations get the NOUO NO'NO's because technically, the antenna is exceeding the 3 meter limit. Although the loading coil is short and electrically making up for the carrot stick's lack of length to wavelength, due to the fact that it is radiating, it becomes a part of the overall antenna length. This is why its best to have 1 loading coil period, be it at the bottom, middle or top and sized accordingly so as to make the entire antenna meet the 3 meter maximum length limits.

An inspector may consider your added 4' whip (if it has a loading coil as well), to be exceeding the maximum 3 meter length, even though when taking a measuring tape the whole assembly measures under 3 meters. If you are going to experiment with the whip without any added loading coil, I dont think you will have any issues passing an inspection as long as your meeting the 3 meter maximum length limit. (antenna/lead/ground)

I have been working on a "Tesla coil" type antenna system where the radiating element has no physical or electrical connection. Using Tesla techniques to further maximize the RF energy transfer.

I say experiment with the 4' whip and measure the results. But I think that if it has another coil, that might dampen results than if it did not have a coil at all on the 4' whip. Your right, experimenting will be the only real way to find out how well the design is going to work. Theory on paper is ok..but in the real world..theory on paper and real world application RARELY mesh.

Good luck! Let us know the results! 🙂

RFB

"My thought was to mount the antenna on a metal post driven into the dirt, with some ground radials just under the turf at the base of this post. The idea was to just use a short ground jumper to the metal post..... Is that legal or does the support post count as a radiating element? Could I just use a coaxial for ground to make in not "radiate"?

There has been a huge (no kiddin') number of messages and discussions in these forums on the supposed legality of above surface ground connections. I suggest spending some time to peruse those, and draw your own conclusions.

I have a question, though ... Why do you think a CB antenna would out-perform your current setup? I.e., I'd think the loading coil winds in the CB antenna will decrease as much signal as the loading coil winds in your current setup ... maybe even more because skin effect of the copper tube helps.

Poses an interesting situation though, and I hope you'll continue. I've thought along similar lines and would like to find out what happens.

i live 8 miles from FCC in lakewood. im in arvada. if you can, please put your contact in touch with me at kc8gpd at hotmail dot com

i'd like to run some site designs by him and get his opinion.

i may eventually wind up living with my g/f in her condo and as you know that means no outdoor antennas and in this case she has a 10 ft high attic at the peak of roof.

i would need guidance on how to properly install it if/when that should happen.

Thanks RFB:

Double loaded sounds like an apt description. It makes sense to have one loading coil in a well designed antenna. However the idea was replacing the top 4' of a single Freq antenna with a "firestick" to down tune it, basically making a single Freq antenna good for two Freqs.

As far as cheating or making it electrically longer I figure all is far game, as long as the total length of the stick is 3 meters or less. However I defer to your opinion. I don't know FCC regs, but do know other CFR's. They are always written by lawyers to be interpreted by lawyers. That makes me think of continuous or helical wound antennas. My base line design for 1380 Khz is 1-1/2" PVC (1.9" OD) wound with 320' to 560' of wire. The PVC part will be 9' and the top a 10" adjustable/telescopic aerial. The on-line calculators vary, but one design I found on line was 4 meters long. PVC wound with 320" of wire. On top was 16" rod (adjustable) topped with a capacitance hat, four 10" radials. This being Europe they get away with it. It is mounted on a roof and apparently transmits well. I think I might buy a stick of PVC and wind it with some stranded insulated 20 awg I have and test it.

Thanks for the help. I will let you know what I find with both ideas. Cheers

Helical wound or continuous loaded antennas have come up in discussions here. The prevailing opinion is that the total length of the wire wound around the form is counted as the length of the antenna.

Probably because the coil is the radiator.

The only way to know for sure would be when the Field Inspector stops by to see what your doing.

The prevailing opinion is that the total length of the wire wound around the form is counted as the length of the antenna.

Well here is where we get into the "gray" area of things. Prevailing opinion regarding "total length including antenna, lead and ground shall not exceed 3 meters" means nothing....the prevailing opinion that is.

The rules do not specify that your 3 meter total length cannot be comprised of a coiled wire antenna. By all means let the prevailing opinions properly post where in the rules it states that the use of a coiled wire antenna that is within the 3 meter length limit is not permitted.

Most shut downs have taken place due to either excessive power, or an antenna exceeding the 3 meter limit. No where in ANY NOUO or NAL is there evidence of a station being shut down because they used a 3 meter long coiled wire antenna. If coiled wire antennas were not legal to use...why does the FCC allow CB shops and others to sell coiled wire CB antennas and loaded whips?

Now the most recent shut down (KENC) was due to the fact that they parked their Rangemaster up on a tall pole and connected the ground lead, though short, to this tall pole to serve as the ground connection. This setup most certainly exceeds 3 meters! Since the short ground lead out of the transmitter case is connecting to a tall pole, that technically makes that ground connection LONGER than the short lead out of the transmitter cabinet, in turn causing that mast to become part of the antenna system even though it is the ground part of the antenna system.

As we have seen, even with the so called "approved" choke coil installed, it still did not meet the 3 meter limit. Why?...this is extremely simple. The mast is still serving as a "ground". The rule clearly states that TOTAL length of antenna, lead and ground lead "if used" shall not exceed 3 meters. In the eyes of the inspector, as well as any sane person...the entire setup was exceeding 3 meters, choke coil or no choke coil.

Disconnect that ground lead completely and then you are well within the 3 meter limit even up on a tall pole or mast. However that also opens up to interpretation if the power and audio grounds would serve as part of "ground lead" for exceeding 3 meters.

The only way around this would be to have 2 isolated grounds inside the transmitter. One for the RF, and one for the power and audio circuits. They cannot be of the same ground potential. At that point the audio and power connection wires, even if they ran 500 feet, would not be a part of the RF ground or antenna system, and then you would never exceed the 3 meter antenna, lead and ground lead limit unless you connected the RF ground lead to a long wire or to the tall mast.

My contact in the Denver FCC field office specifically stated that it is best to install these things either at the surface of the ground, or on a very very short pole not any taller than 2 or 3 feet. The more pole you mount that transmitter to, the greater length the ground lead will become, choke coil or no choke coil, it is still the ground lead connection even through the pole. Thus you must reduce the length of the radiating element to make it all fit within the 3 meter limit.

Isnt all this fun?! Interpretation...opinions...it is fun sorting through all the thousands of different interpretations even those of the FCC field engineers. Each one of them will have a different interpretation of any given installation. They, like lawyers, can all look at the same letter of the law and all will have a different interpretation to that one law, making all of this very confusing.

Its best to stick with what the letter of the law states, no more than 3 meters total...antenna, lead and ground lead...period. If you mount up on a 20 foot mast, then do not use the ground lead at all. If you mount on a 1 foot pole, you can safely use the ground lead and slightly shorten the radiating element to make the entire length within the 3 meters.

If you construct an antenna system comprising of a coiled wire that fits within 3 meters...your abiding by that 3 meter total length limit physically. Electrically your basically "cheating" to make up for that short length to long wavelength. Again NO WHERE in the rules does it state that a coiled wire antenna that is crunched down to fit in 3 meters length is not permitted. However I am always open to correction and will gladly stand corrected if someone merely posts the rule stating a coiled wire antenna measuring 3 meters is not allowed.

RFB

RFBurns wrote "A sensing loop consisting of 1 turn of #22 AWG wire around the base of the radiating element inside a rubber washer and feeds an op amp to send a DC voltage proportional to the emitted RF signal for metering on my antenna tuning control panel."

Does your pickup coil respond to the antenna current, the field induced by the loading coil, or both?

I ask because the results of my experiments with a resonant base loaded antenna system indicate that the maximum real power delivered to the system does not happen at adjustment points of maximum voltage or current at the system feed point. I have observed that the maximum real power occurs at an I to V phase angle of approximately 20 degrees. I attribute this to a possible reactive component in the output Z of the transmitter and the slightly reactive antenna system Z giving maximum real power delivered to the antenna system may be the result of a complex conjugate match between the two. Have you noticed this effect?

Neil

Nice summary here:

FCC Enforcement Actions in the AM Band

None of the nuances discussed here appear in the noted enforcement actions, the FCC's approach is pretty straightforward.

No high powered devices, no long leads, ground or otherwise.

The prevailing opinion is that the total length of the wire wound around the form is counted as the length of the antenna. was never mentioned in the enforcement actions.

No where in ANY NOUO or NAL is there evidence of a station being shut down because they used a 3 meter long coiled wire antenna. is certainly verified in the above-linked NOUO review.

EDITOR'S NOTE - please remember that we ask participants to post only their own work. If "My contact in the Denver FCC field office" wants to post here then he or she is welcome to do so, if not then please state the information as your own and please drop the references to alleged sources who are not participating and who cannot be contacted or verified.

If "My contact in the Denver FCC field office" wants to post here then he or she is welcome to do so, if not then please state the information as your own and please drop the references to alleged sources who are not participating and who cannot be contacted or verified.

Not a problem. I was merely using my contact as the reference for my own installation as it is built based on the information given to me. As such the statements given thereafter are from my own experience and observations and not necessarily only from that reference.

RFB

Does your pickup coil respond to the antenna current, the field induced by the loading coil, or both?

The sensor coil placed at the base of the 9.2 foot long, 1/2 inch OD copper pipe measures the field induction. Antenna current is measured across a small "grain of wheat" light bulb placed between the output of the loading coil and connection to the copper pipe. The current measurement is not ran to the metering cluster...yet. Isolation is quite tricky at this point due to the fact that to get a direct current reading, a direct connection is required and adequate RF blocking must take place here before going to circuity to drive the meter circuit. Im working on this aspect of the system, however its quite difficult because I am also trying to keep any induced RF off of the internal wiring harnesses that may end up on any ground leads or be induced into them and travel into the studio as well as have minimum negative effects on the resonance of the whole system.

Not so much of a problem when your working with 1Kw or more, but on a flea power system where every fraction of an ounce of resonance is crucial for maximum performance, a mere wave of the hand within 3 feet of the radiating element and loading coil can throw off the whole works.

Plus the box is rather limited in space at this point because as I have kept adding and adding things to the final build, space got filled up rather quickly. Time for a bigger weather proof box! That is also on the drawing boards.

I ask because the results of my experiments with a resonant base loaded antenna system indicate that the maximum real power delivered to the system does not happen at adjustment points of maximum voltage or current at the system feed point.

Indeed. However when using both the internal measuring points and referencing to external field strength instruments, a "zone" can be clearly observed where the maximum system resonance takes place across the tuning range, thus the calibration of the internal metering is set to match that of the field strength measuring equipment at the peak resonance point indicated by said external measuring gear.

The offset is not that much actually..considering the power levels and resonant element length involved. However because of that fact...too much tapping and sensing and measuring schemes even with adequate isolation and bypassing begins to basically load down the system and at that point it all becomes rather pointless.

But yes I have noticed the effect in various designs I have built and they all have their own unique properties and need their own unique solutions, which makes it all worthwhile in the spirit of experimentation.

RFB

However I am always open to correction and will gladly stand corrected if someone merely posts the rule stating a coiled wire antenna measuring 3 meters is not allowed.

Well than that applies to any loading coil. Even the most basic base loading coil is about 80 feet of wire. Mid or top loaded coils for the upper end of the BCB might be over 300 feet. I guess it comes down to who are you bothering or who is complaining, and they can't get you some other gross rule violation.

The ridiculous extrapolation of this helical wound antenna, is instead of using 1.9" to 3.5" Dia. PVC, use a former of a foot or more in diameter, by 9.8 feet tall. Theoretically I think it would violate some basic rules of thumb for a helical antenna design, like Dia to height ratio, something like 5. So in theory it could be 9.8/5 or almost 2 foot diameter! However to make it work you need welding cable to wind it, because there is a wire/Dia. pitch requirement. I sketched the antenna out. There are some errors in this rough sketch and calculations. The ground resistance says 40 but the calculation is on 10 ohms (cut and pasted the right data in after the fact). The dia of the PVC should be 1.6" but a 1-1/4" PVC or 1.9" Dia would be better.