Hairpin Monopole Antenna

I just plugged in 1.64 Mhz and come back with 17 feet, so too long for part 15 AM in the broadcast band

A folded monopole antenna above ground is, according to the theory of images, half of a folded dipole. Its input impedance is half of that of a folded dipole. It usually consists of two monopole elements (of equal diameter) connected together at the top. One element is grounded at the bottom. The other element is driven by an RF source at the bottom. It is a shorted length of transmission line less than a quarter wavelength long.

The short folded monopole works like an ordinary monopole the same length, but with four times the input impedance, with an inductance connected across the antenna input. The inductance comes from the shorted transmission line that makes up the antenna. The effective monopole works like the two monopole elements shorted together at both the top and the bottom.

The folded monopole got its reputation as an effective short antenna because of the General Dynamics Hairpin Monopole, used with a Collins KWM-2 transmitter, during the Vietnam War. An automatic tuner was able to change frequency anywhere within the HF range quickly to avoid jamming or interception by the enemy.

Unfortunately, the hairpin antenna actually works very poorly when it is electrically short. The four times multiplication of the input impedance suggests that the radiation resistance should be higher than an ordinary monopole the same size. Actually, the opposite is the case. The series-equivalent input resistance is, indeed, multiplied by four, but the series-equivalent capacitive reactance is also multiplied by four. For a short antenna, the reactance is much bigger than the resistance. So, when it is all worked out mathematically, the result is that the parallel-equivalent resistance is multipled by four. Increasing the parallel-equivalent radiation resistance results in a decrease in the efficiency of a short antenna, just like reducing the series-equivalent radiation resistance. But, it's even worse than that. The shunt inductance produced by the transmission line causes a further reduction in antenna efficiency. So, for a short hairpin antenna, the efficiency is nearly zero.

It is quite another matter when the electrical length of the hairpin antenna approaches a quarter wavelength. At resonance, the monopole mode of the antenna has no reactance, and the parallel-equivalent resistance is the same as the series-equivalaent resistance. So, at resonance, the radiation resistance is actually increased by a factor of four. Also, at resonance, the transmission line mode of the antenna has the same performance as a quarter-wave stub, and the shunt reactance across the input of the antenna disappears.

A quarter-wave hairpin antenna can be a lot shorter than a quarter wavelength in space. This is because the hairpin is a slow-wave structure, where the propagation velocity of the wave along the antenna decreases with greater separation of the monopole elements. So, a resonant hairpin can be significantly shorter than an ordinary resonant monopole. But a resonant hairpin is not nearly as short as the antenna length limitation in Part 15 AM.

Well, I can't speak for everybody here, but I like learning things even if they may not apply directly to part 15 AM.

Obviously true that it would be too long for part 15 AM on the BCB. But there could be other possibilities.

But if someone wanted to play with the shortwave part 15 in the 22 meter band, that's about 13.5 mhz which would make a hairpin antenna for it only a couple feet long. That'd be handy if one lived someplace where neighbors didn't like bigger antennas.

Also it could be made pretty portable and might be of use if one wanted to use it as an STL for a part 15 AM station for doing something like "live coverage" of events. Or if someone lived on the edge of town and had a nice hill on the property, but it'd take running 1/4 mile or so of cable to put their transmitter up there? Batteries and a shortwave reciever and SW Part15 could be a possible way of doing it. Maybe use an RC type tone to turn the xmitter on and off that also could go out over the SW. That way only the reciever would need to stay powered up all the time.

For that matter, use it to feed audio to multiple transmitters that are at good locations around a town, but not so close to each other that their range would overlap and "beat". Maybe not optimal, but still probabaly better than using something like equalized phone lines and it'd be considerably less expensive.

I wouldn't want to rely on it for anything commercial, since anybody else could decide to set up a part 15 SW on the same frequency since if I recall right the slice of the 22M band is too narrow to have more than one station on it in an area and have enough bandwidth for audio. But for experimentation/hobby it'd definitely at least be interesting.

Another thread HERE mentions the technical specs for the 22M part 15, and the power allowed is considerably more generous than is allowed on the AM or FM (I'd guess because there isn't the same chance of interfering with commerial licensed broadcast stations like there is on AM and FM).

The restrictions on things like antenna height, grounding and etc are also much less restrictive.

I'd think the most likely use would be for "live coverage" within a reasonable local range, though. A directional antenna on the reciever/studio end aimed in the rough general direction to cut down any interfernce coming in on skywave, maybe. I believe I recall seeing some ham designs for 20M antennas for use when space is limited that were reasonable sized. The noise off the shortwave band might be too much for use as a 24/7 type STL though.

Lee runs part15 SW on 13.56 Mhz. Maybe he can give us an idea what the actual practical range and sound quality are like in a city/town.

But a physically small antenna (that could be made very durable) for that range might be encouraging to city-dwellers whose landlords or neighbors might not approve of a big antenna. And the possibilities for a very portable antenna that wouldn't take long to set up that could be used with part15 SW for "on the spot local news coverage" to the studio of a part15 AM is a tasty thought.

Daniel

L. B. Cebik, W4RNL writes:

"The coaxial folded monopole lies at the base of the coaxial collinear array and a number of other antennas. The difficulty that designers and analysts have with the antenna lies partly in our inability to effectively model the relatively simple device. However, in principle, the antenna is as easy to understand as a simple folded monopole. "

Read much more here:

www.cebik.com/model/cofd.html

Experimental broadcasting for a better tomorrow!

Daniel,

Thank you for mentioning the interesting thread about Part 15 HF operation. This thread is in the "other" Part15.us Forum about items for sale. "mram1500" gave a long and informative technical report about operating on 13.56 MHz. He said that the field strength limit is 10 mV/m at 30 meters. I thought it was about 16 mV/m. My latest copy of Part 15 is a few years old, so I looked up Part 15.225 using the www.fcc.gov web site. The field strength limit is 15.848 mV/m. This corresponds to about 2.5 mW of radiated power from the antenna. This is pretty good by Part 15 AM broadcast band standards, unless you are using a whip and mast antenna. It happens that the atmospheric noise at the upper end of the AM broadcast band is actually lower than at 13.56 MHz, but this is only of interest to people living in rural areas. Urban man-made noise, however, is a lot lower at 13.56 MHz. The information I have is that urban man-made noise is typically about 150 uV/m in the AM broadcast band and only about 4 uV/m at 13.56 MHz. This allows the possibility of considerably greater range using this Part 15 HF channel.

mram1500 said that there is hardly any ground wave propagation at 13.56 Mhz. This is not quite true. In fact, a significant portion of the propagation you will be able to get in this channel will be ground wave. If your antenna is at the level of the earth, that is about all of the kind of propagation you will get. Ionospheric propagation at such a low power level is really out of the question. mram1500 mentions QRP operation on the Ham bands, where contacts were achieved at astonishing distances using a few milliwatts, but this sort of thing takes a lot of time and patience. It also requires the use of Morse code. If you want to do two-way communication on HF, get a Ham license. The FCC has eliminated the only significant barrier to entry to amateur radio by eliminating the Morse code test. A lot of people had trouble getting up to speed in Morse code, but the thechnical examination was easy in comparison.

The only things you can't do with a Ham license is broadcast, and play music. So, being able to broadcast is a significant advantage in using 13.56 MHz. As I already said, ionospheric communication is out of the question, since the skip distances are simply too long for such low power. So, ground wave is an important component of the signal. Ground wave attenuation is about 15 dB higher at 13.56 MHz than at 1700 kHz. Fortunately, at 13.56 MHz, there is a gain when the antenna used for transmitting is elevated. Since the FCC limits power by field strength limits, there are no limitations on the antenna structure. It is necessary to elevate the antenna (which must be vertical to get ground wave propagation) about 150 feet to overcome the increased ground wave attenuation at 13.56 MHz. It is difficult to calculate range with the comlex propagation that occurs in this channel. I think that, with an elevated antenna, about five miles is possible.

Now, who can you broadcast to? Everybody has an AM radio, but a lot fewer people have shortwave radios. Neverheless, shortwave radios are consumer products, and there are plenty of people who have them. I think that it is best to use AM rather than SSB in order not to shrink your potential audience even further. While a lot of shortwave radios have BFOs, they usually work very poorly. SSB has been called "Silly Sideband" for a good reason: It sounds really lousy! Don't even think about transmitting music using it.

scwis,

The coaxial folded monopole antenna is similar to a hairpin antenna, except that, instead of the antenna being an open-wire transmission line stub, it is a coaxial transmission line stub. Since only the outer cylinder, and not the center wire, radiates, (since the center wire is shielded by the outer cylinder) there is no radiation from the center wire. Because only one of the two conductors radiates, there is no multiplication of the input impedance, as happens with the hairpin. At resonance, when there is no shunt reactance due to the transmission line mode of the antenna, the antenna performs about the same as a cylindrical antenna the same height and diameter.

The article you linked was very helpful in understanding the coaxial folded monopole, and it also demonstrates the difficulties involved in using antenna modeling programs. The NEC program cannot model the outer cylinder very well. The best it can do is approximate the cylinder as several vertical wires arranged in a circle. This is not necessarily a bad thing, because real coaxial folded monopoles are often actually constructed using several vertical wires arranged in a circle, with a conductive structure that shorts the wires together at the top.

In an article in the September 2001 issue of the IEEE Transactions on Broadcasting, Valentin Trainotti wrote about the coaxial folded monopole antenna for use as a short antenna for lower-powered (in the kilowatt range) licensed AM broadcast band stations. I had mentioned Prof. Trainotti previously in the EH Antenna thread. His niche is small AM broadcast band antennas, and he is the leading expert in the world in this field. In his article, Trainotti uses an antenna modeling program, and he models the outer cylinder as eight verical wires, with eight cross-wires joining the vertical wires to the central conductor. The results he obtained are truly amazing. They are so amazing that I have some doubt that they are actually correct.

When the height-to-wavelength ratio is low, the input series resistance is practically zero, as in the hairpin antenna. At resonance, the input resistance goes very high. This is unexpected, since there is no impedance multiplication effect in this type of folded monopole. Right after resonance, there is a minimum in the resistance curve, with some shunt capacitive reactance, where the resistance can be coupled, by an antenna tuner, to a 50-ohm transmission line, with a Q less than 10. This resistance minimum occurs, at 1700 kHz, when the antenna height is about 46 feet, provided that the diameter of the cylinder is about twice its height. 46 feet is far too long for Part 15 AM, but it's a nice short length for licensed broadcast stations. It avoids some antenna painting and lighting regulations.

I wrote to Prof. Trainotti, remarking that it appears that his folded monopole works better than a fat, short, cylincrical monopole with the same dimensions. Trainotti's reply was that the cylindrical monopole is not an electrically short antenna at all, but a quarter wave resonant antenna.

I think that the characteristics of the cylindrical monopole are an open question in antena design. The computed results need to be supplemented with actual data. But what does this have to do with Part 15? It is rather difficult and expensive to use a licensed broadcast station for antenna experiments. The experiments can't be done under 15.219, but they can be done under Part 15.209, where a very severe field strength limit applies. To use more power, a special temporary authority could be obtained under Part 15.7.

In anser to Greg_E's dilemma, and the technical follow-up, if a monopole is half a folded dipole, how about inserting loading coils somewhere, maybe halfway up, in both legs? Keep the 3-meter height, just make it electrically longer.