I’m working on an AM transmitter that modulates a digital 1MHz signal by turning it on and off. I need to drive a short (6″ to 9″) antenna with this and need to know how to optimize transmission. I’ve read about using antenna loading coils and found an online calculator that told me to put 10mH in series with a 9 inch antenna but when I do that I still hardly hear anything on the AM radio 5 feet away. Anyone have any ideas?
Probably have some mean resistance and reactance there
If you really mean 10 milli-Henrys (which would be about right for a 9″ raditor by my calculation) then your signal is going through so much copper there isn’t any left to transmit.
Depending on how the coil is constructed the reactance could also stop the AC signal dead.
Not sure a 9″ radiator will really be doable – you might try a 9″ helix 🙂
Experimental broadcasting for a better tomorrow!
It’s a stinky antenna.
The
It’s a stinky antenna.
The original post may be a spoof, but I bit and cranked up EZNEC and a spreadsheet I have for calculating resonance.
9″ antenna: 2.4063 pf
Inductance required at 1 MHz: 10526.673 uH
If a “good quality” RF choke is used like the JW Miller 10 MHz part number 4672-RC. The DC resistance is 50 ohms. The self resonant freq is .82 MHz. Core loss is not specified.
Assuming a total coil + ground loss resistance of 100 ohms, EZNEC predicts the antenna gain to be -53.65 dBi (miniscule signal unless power input is high).
Also very important is the low self resonant frequency of the coil. At .82 MHz the self capacitance of the coil is about 3.8 pf. This is GREATER than the antenna capacitance. This will make the radiated signal even worse than EZNEC predicts due to the voltage dividing effect of the coil capacitance and the antenna capacitance.
There, that’s my $0.02. I hope this doesn’t become a big thread! It is interesting to illustrate how a really, really short antenna compares to a really short 3 meter antenna 🙂
No spoof – just tight constraints and limited experience
Thanks for the comments. Antennas was one subject not covered very well in school so I’m learning on the fly. Can you point me to practical equations and/or modeling resources? Does a short antenna look like a capacitor (with resistance) to ground? Is EZNEC a software tool I can buy or find on the internet? Is there a way to make the antenna ‘longer’ while still fitting in a 9″ length (I’m assuming serpentine, coil, or helical shapes don’t buy a lot)?
Short AM aerial
EZNEC is available as a free demo for download though the number of elements in the antenna is limited, yet for what you need it is worth a try.
A short antenna compared to a 1/4 wave ground plane or 1/2 wave dipole looks capacitive. The antenna will radiate but the major problem is to match it properly to your transmitter to get a usable power transfer. That’s why a loading coil is useful for short radiators. A decent resource for hobby type antenna theory is the ARRL Handbook but this addresses mainly antennas of appropriate length to radiate efficiently.
The success or failure of your project may hinge upon what you are trying to do with this transmitter. A 9″ antenna will radiate a signal and, though perhaps not optimized, it may give the range you need depending upon the transmission path, interference, and the receiving equipment.
Such a short antenna will be very sensitive to things nearby in terms of its impedance and maintaining a tuned system with a resonant loading coil will be difficult. Just a guess, but you should be able to get a range of a few tens of feet with a 100 mW. rated transmitter but I doubt you can get much more and this is only based on some experiments I have done and it is not scientific at all. Your results may vary considerably from this guess.
Neil
Short antennas and switched transmitter output stages
There is no way to “lengthen” a short antenna. A small antenna will necessarily have low efficiency. The theoretical maximum efficiency is proportional to the effective volume of the antenna. Some proponents of bogus antenna designs claim high efficiency for small entennas. The claims are based upon some antenna element that the proponents do not count toward the antenna length, such as a long ground lead or a long transmissin line. The complete theory of small antennas is given by H. A. Wheeler,”Fundamental Limitations of Small Antennas,” PIRE, December, 1947, p.1479. A small resonant loop antenna would probably be easier to construct than a small rod with an enormous loading coil. Using the 10 mH loading coil would probably not work, anyway, because of its high self-capacitance, which would require the loading coil to operate above its resonant frequency. It might be possible to use a much smaller low-frequency inductance than 10 mH, and operate the loading coil just below its self-resonant frequency, giving a higher effective inductance at the operating frequency; but such an arrangement would give the loading coil a very low Q.
Using a switching output stage for the transmitter would be a good idea, if it can be made to work. Pulse-width modulation of a pulse train operating at the carrier frequency can produce nearly 100% efficiency, but the switcihng time has to be very short. A sufficiently short switching time would be difficult to achieve in the AM BCB.
REALLY short AM transmit antenna
Hi I’m new to the group and I’m impressed with these comments to a difficult problem. The 10mH inductor probably is acting like a resistor. A 9″ long wire at 1MHz has really low radiation resistance so when you put the two in series, all your power goes into the loss of the inductor. A better solution might be to choose an inductor design primarily with high Q, and the inductance not necessarily to resonate exactly with the 9″ wire. Then you could resonate this parallel combination with an added tuning capacitor. Whatever you do you should try to maximize the 1MHz RF voltage at the 9″ wire and this can be done by either tapping the coil or putting another winding of less turns on it and driving it with a low impedance 1MHz source. A capacitive antenna in this case is better than a loop because the receiving antenna is still in the near field (~160′ away). Another interesting paper by Harold Wheeler is, “The Radiansphere Around a small Antenna” (August 1959 Proc. IRE). What is the application?
Welcome AA1LL
Nice to have you aboard!
Just a little theory and a mini tutorial to get us all up to speed. The standard model for a short (3 meter) antenna used for part 15 AM shows a capacitor in series with a resistor. This resistor is the radiation resistance (Rr for my discussion) and it is the power dissipated in this Rr which carries the freight.
So, we attempt to maximize this power. To do so we need to negate the effects of the series capacitance (XC) and we do so by means of a series loading coil (XL). At resonance, the XL in the coil cancels the XC in the antenna and the transmitter sees only the Rr.
Now, the bad news. The Rr is on the order of .1 ohms or so but the AC resistance of the coil is tens of ohms. This forms a voltage divider and most of the voltage and power is lost in the coil resistance and your point about using a high Q (low resistance) coil addresses this.
But, another big factor is the ground resistance which dominates the situation. We try to do our best to minimize this but it remains the primary cause of loss of power in a legal part 15 AM system.
Your comment about maximizing the voltage to the antenna is correct but even in doing so most of the power is lost in the coil and ground resistance. The maximum voltage is produced across the L and the C in a series circuit at resonance and, depending on the resistance in the circuit, these voltages can be huge. Nonetheless, maximizing the voltage at the far end of the antenna radiator is the correct approach. I can get a NE-2 neon bulb to light as I hold one lead in my fingers and touch the other to the antenna (far end where the current is zero) of my less than 100 mW transmitter system.
73 en tnx fer QSO.
Neil
real short antenna
A real short antenna works better at a real short wavelength. Have you considered using an FM part15 transmitter? Sounds like a perfect application for it. If you are dead set on using AM,try the 13.56mhz part15 segment,I use it here. Either will give you better range over the AM/BCB part15 band. There are other part15 bands around 49mhz and up into the ghz range if you want to be obscure. Enjoy!
73,Lee,N3APP,CET,FCC GROL,ARES,RACES,SKYWARN
EchoLink node #9534
RadioFreeErie AM/FM/SW
http://www.freewebs.com/wilcomlabs/index.htm
13 MHz operation
Lee,
You mentioned operation on non broadcast bands. This is great for the experimentalists but what expectation is there that there will be actual listeners. Perhaps that is not the objective.
I don’t mean this to discourage anyone, especially you who have contributed so much to this board, but realistically I must ask why do this? Educate me.
Neil
bands and range
Neil, the post was for a digital signal in the AM band. I dont know who could percieve this as any form of broadcasting as there are no recievers or potential audience. He seems to just want to transport a digital signal a short ways,I simply gave the sensible alternatives. Besides,I have a couple SWL’s listening to my 13.56 feed,so dont sell it too short. It has the best range of any part15 band and I consider it to be a broadcast band as anyone with a SW set can tune it in. I guess its all in how you interpret things,my mindset is fairly broadbanded,LOL. 73,Lee
13.56 MHz questions
Lee,
I would like to know more about your SW operation. My questions are:
1. How do you verify that your field strength is within the limit of Section 15.225?
2. Is your antenna horizontal or vertical? (I want to know if you are trying to use groundwave or skywave.) What kind of antenna do you use?
3. How distant are your SWL listeners? (Do they hear you by skip?)
The high field strength limit for 13.56 MHz makes this “band” seem promising to me compared to other Part 15 modes.
REALLY short TX ant. and what freq. to use
The question about how the signal propagates is a good one. Even at 13.56MHz it sounds like the receiver is still in the “near field” of the transmitting antenna, so it’s not really a propagation issue is it? Can we consider the transmitting antenna and the receiving antenna like two conductors that are capacitively coupled? Then I guess you have to figure out where “ground” is and how the transmitter “ground” gets connected to the receiver ground. But the point is, maybe it can be treated as a static electricity problem rather than an electromagnetic one since everything is close together compared to a wavelength.
13.56
Ermi,
I ran a “hamstick” mobile type vertical made for 20 meters up about 25 feet. My “listeners” are some local hams within a few miles using yagi’s. Power output is about .55mw. Definately ground wave. Ike took down half of my antenna farm and that one didnt survive too well. I am going to try a dipole next time. I am looking for a good SW portable radio to do some more testing when I get it back up. 73,Lee
More about 13.56 MHz
Thanks for the information about your setup. Sorry to hear about Ike. That Ike took down ony half of your antenna farm, and not all of it, is a good sign. Maybe your roof still keeps the rain out!
When you repair your 13.56 MHz system, please try a horizontal dipole, because it would have very little radiation in the horizontal direction (which is the direction in which the field strength measurement is made), but a lot of radiation skyward. Maybe it is possible to get enough power in the sky wave to be heard a long distance away by skip.
I see that your strategy for meeting the Section 15.225 field strength limit is to apply a low enough power to the antenna to assure compliance. This approach would not work for horizontal polarization, however, because it is not easy to calculate the field strength in the horzontal direction as a function of antenna input power. A means of measuring field strength would be necessary.
13.56MHz vertical or horizontal
I didn’t know people were using 13.56MHz to communicate digitally. Do you use TCP/IP? I am learning more every time I read something at this website, and would like to contribute: Lee, if you want to generate a ground wave signal you need to be vertically polarized. A horizontally polarized wave will have its electric field greatly attenuated (like, pretty much shorted out) by ground conductivity.
73 DE AA1LL
13.56 MHz range
Using the expected ground attenuation and daytine noise level at 13.56 MHz, I calculated that, using groundwave, the range should be about 1/3 mile in urban areas, and one mile in rural areas. Using Yagis at the receiver would increase the range by maybe a factor of 2 to 2.5. Since HF Yagis are likely to have only a few elements, I don’t think the range increase would be even as high as 3. From Wilcom Lab’s post, it seems like his listeners hear him from a greater distance than I calculated.
I am assembling a 13.56 MHz station, and I am going to use a horizontal half-wave dipole about 12′ above the ground, fed off-center through a 300 ohm twin-lead. Off-center feed makes it easier for me to drive the dipole from my house.
It is unlikely that I can get enough power gain using a horizontal dipole to work skip, but I will use horizontal polarization just to see what happens.
13.56 MHz is certainly a legitimate frequency to use for Part 15 broadcasting. S.W. receivers are consumer products, and millions of people own them. Of course, more highly-motivated listeners are needed than on the AM and FM broadcast bands, but the range should be greater.