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i have some 300 to 75 ohm baluns for tv use and a FM turnstyle type antenna and a v/u rooftop yagi.
i want to use these parts to build a transmit dipole.
Total posts : 45366anyone have any designs to point me to? would like to make it wideband/no tune design.
Total posts : 45366Tuned would work better.
With our low-power levels tuning doesn’t really matter though.
I think on my 1/4 wave if you run under half-a-watt you don’t have to tune the antenna.
Total posts : 45366Tuning has nothing to do with power level except the less power you have,the more important it is to tune it well. It certainly does matter. I tuned my FM antenna and quadrupled my range. My AM wouldnt go 100 feet until it was tuned properly,now it goes miles!!! Tuning ALWAYS matters and is what separates the men from the boyz. Take the time to tune it well and you shall be rewarded.
Total posts : 45366My experiences have indicated that at least on FM, tuning is not as critical as you might think.
I know, that flies in the face of logic. I’ve experimented with dipoles, using the same one right across the FM band (so SWR was not optimal for most frequencies) and my range was pretty constant right across the band. SWR wasn’t super high, mind you, but in some cases significantly greater than 2:1.
I suspect that there are plenty of other factors that affected my range that dwarfed SWR and antenna tuning. At least in my particular case.
And sure, if your antenna is waaaaay off its resonant frequency, tuning it will increase range. Depends on your particular situation. As usual, one size doesn’t fit all.
AM is an entirely different matter, and I’ve found that tuning the antenna did make a noticeable difference (probably because your resonant frequency range is tiny at lower frequencies, and SWR can approach infinity rather quickly outside that razor sharp range). But even there, improving my ground was a far more important factor.
Total posts : 45366This isn’t exactly on topic, but it may be useful.
I have several Part 15 FM transmitters.
The one that gets the most use is a North Country
Radio MPX-96, that a Part 15 friend built for me.It has a 50 ohm output. I have a bunch of attenuators
in the antenna feed line. Right before the antenna,
the output is around one mW.The coax cable goes to a 75 to 300 ohm transformer and
feeds a receiving dipole from Radio Shack that is constructed of
300 ohm twin lead. (The cheap transformer probably adds
more attenuation.) You might want to use something better.Although my FM transmitting set-up is not built for covering
a large area (It’s actually on the first floor of a three story house)
– where it does transmit – there are very few nulls.The Part 15 station sits on a computer table. The transmitter is also
on the table. There are a lot of wires connecting everything together.
I tried to neatly hang the dipole antenna on the wall. It kept falling
down and getting tangled up in other wires.I ended up putting it in some plastic plumbing pipes from the hardware
store. If you can picture this, the whole thing looks like a letter “T”
made out of the pipes. One pipe goes straight up from a base I made,
where it feeds into a T connector. Then each side of that connector
has a pipe going out of it. The legs of the dipole are in these pipes.
You can probably picture how this looks. It’s just a big letter “T”
made out of pipes with the dipole in it.It’s tall and almost to the ceiling, so it clears the turntable, mixing
board, the big station clock, the on-the-air light, cd player, 3 tape decks, the
audio processing equipment, and the transmitter.And it doesn’t look too bad. i plan to paint it some nice color soon.
Anyhow, the bottom line is that it protects the dipole, and the radiator
sections are way above the station equipment.With this set-up in use, Part 15.239 compliance is not a worry. And anybody
could put a tuned dipole in there. (I hope the dialectric content of the pipe
isn’t a problem, though. I have no evidence that it is with this set-up. )Bruce, DOGRADIO STUDIO 2
Total posts : 45366i was thinking of hacking the boom on the v/u to 1/4 wavelength and sticking the folded dipole on the end and feeding it with a 300/75 balun.
or make boom 1/2 wave and put a reflector on it as well.
maybe make some arrangement where i can vary the length of the folded dipole for the top, mid, and low bands.
Total posts : 45366I can’t comment on your proposed design because I just now opened the book to learn about the folded dipole.
I am looking in Radio Handbook 20th Edition by William I. Orr, W6SAI, page 26.11…..
There are many different variants shown, and it looks like your 75/300 balun would be right assuming your transmitter output is 75-ohm, which I guess is normal for VHF. The classic feed line to a folded-dipole appears to be 300-ohm line.
My FM receiving antenna is a folded dipole made 100% from 300-ribbon cable, and must be wide-band because it came with the radio for general FM reception. I think it’s fed by a 75/300 balun.
The folded-dipole appears to be entirely horizontal…. is there a way to have a vertical element within the design?
Also, given the advice that being tuned to frequency is vital to good performance, why would you have a 3-band version of the antenna?
Total posts : 45366it is i have a lpb fms2000 cable modulator continuously variable from microwatts to milliwatts
Total posts : 45366That’s interesting. An FM cable modulator is a legitimate way of putting an FM signal to an antenna, and the flexibility of setting the power level would allow use of a much more experimental antenna than a dinky “certified” transmitter, like this C.Crane that is driving me insane by multi-pathing everytime I move around even though it is only 4-feet from the radio.
More of us should tinker with FM modulators.
Total posts : 45366First, it would be better to use a dipole without the 300 to 75 ohm balun instead of a folded dipole with the balun. A dipole matches 75 ohm coax directly without the losses from a balun.
Otherwise, a dipole or a folded dipole share the same resonant frequency length calculation.
Length in feet = 468/Freq(MHz)
This equation is quite accurate and has been gospel for many, many decades. Search for “dipole antenna”.
As an example, at 98 MHz (center of the FM band) the length is 468/98 = 4.78 ft = 57.31 in.
For this dipole (or folded dipole) the bandwidth is pretty good. It resonates at 98 MHz with an SWR of less than 1.1:1. SWR rises to 2:1 at plus and minus 5 MHz, rising to about 4:1 at the low and high frequency edges of the FM band. If you can narrow down the preferred frequency range, you can make the dipole length to match the center of that range.
A horizontal dipole (or folded dipole) will perform poorly for local coverage due to the effects of ground reflection and ground absorption, which cause most of the radiation to be directed vertically into the sky, and very little horizontally. The very small remaining signal that radiates horizontally is additionally crippled by directionality. There are two horizontal lobes with minuscule radiation at right angles from the lobes.
A vertical dipole performs much better. It has an omnidirectional pattern horizontally and maximum gain at at ground level (0 degrees vertically). This is the best pattern for maximizing signal to the most listeners.
One concern about the vertical dipole is how it is physically mounted. It’s fairly easy to model without the effects of the feed line. Likely the feed line will need to be routed from vertical to horizontal en route to the center of the vertical dipole (maybe 1/4 – 1/2 wavelength from the dipole?). I haven’t done enough research to find this information, but I strongly suspect that running the feed line down immediately adjacent to dipole will pretty much wreck predicted performance. Pictures of vertical FM dipoles sticking out from the sides of broadcast towers in the form of a horizontal “T” typically show the horizontal length to be about the same as the dipole length, which is 1/2 wavelength.
Total posts : 45366There’s a reason a Part 15 FM CERTIFIED transmitter performs so poorly….The FCC doesn’t legally allow any stronger signal than that. As soon as you add an efficient antenna to virtually any Part 15 transmitter, (dipole, folded dipole, ground plane or even a 1/4 wave whip) you will exceed the field stregnth limit. This is especially true of non-certified kits, and “FM modulators”.
Personally, I don’t have a problem with Part 15 stations being over the line somewhat, as long as it doesn’t get rediculous. But with the FCC’s FM limit so rediculously low, it’s too darn easy to overstep the line. For example, according to the FCC’s own formula, if you use a dipole antenna, it only takes about 20 NANOwatts to reach your field stregnth limit! A nanowatt is one BILLIONTH of a watt! If you’re considering 10 milliwatts loaded into a dipole for your “part 15” station, do the math first and you will see just how out of bounds you can get in a hurry! (a milliwatt is 1/1,000th of a watt, remember. A milliwatt is also ONE MILLION TIMES the power of a nanowatt!)
I’m all for good, clean, fun experimentation on Part 15 FM, but before you start using any system that performs considrerably better than a certified transmitter does, you need to know what you’re getting into, and weigh the risks accordingly. Very few operators realize just how miniscule the field stregnth limit is for Part 15 FM. If you comply with it exactly, you might put a good signal into your next door neighbor’s houses. But across the street and beyond — it’s all fringe area! Good luck and have fun anyway! Just be careful!
Total posts : 45366To minimize or eliminate the skewed pattern of the vertical dipole due to the feedline interaction, perhaps a COAXIAL DIPOLE would do.
Our City Public Safety Radio used a type of coaxial dipole for many years on the VHF band. Some called them Hairpin antennas.
Picture a half-wave radiator folded in half and grounded to a 2.5 inch tube which formed the 1/4 wave ground side of the dipole. The feedline runs inside the mast pipe. The mast pipe threads into the base where the radiator attaches to the feedline.
For the do-it-yourselfer, here is a COAXIAL FOLDED DIPOLE made of coax cable.
Total posts : 45366I don’t know what this has to do with a thumb, but a good slogan to help us keep within the rules might be,
If you can’t hear it, you are in compliance.
Total posts : 45366Fully agree with rlkocher. Even the FCC’s Part 15 guidebook explains how attaching a different antenna other than what came with the certified FM transmitter, or an FM transmitter kit, will make the unit operate well outside of the field strength limits.
But since there are NO antenna hight, length, coax restrictions for 15.239 and only the field strength regulation, one only needs to attach the proper attenuation to the feed line to attain that 250uV 3 meters from the antenna. If that is met and maintained, it wont matter what kind of antenna or transmitter your using.
Believe it or not, the FCC field agents are engineers by heart and trade. They, like us..love to be creative and they like to see creative things that are unique and that keep things within the law. The field agent will fully understand your system and any additions to it in order to maintain the 15.239 field strength rule, even if that means having a few attenuators inline on the coax.
The rules only specify that pre-built Part 15 certified transmitters must have special antenna connectors that are not off the shelf common types or permanently attached antennas such as a wire. But ONLY on Part 15 certified pre built transmitters! There is no mention about using different connectors or antennas for kits or home built units. Next to the field strength limit, the certified unit and its own unique antenna/connector is the only other rule to abide by when using pre-built certified transmitters.
The inspecting agent might frown at your modified C-Crane sporting a connector instead of its incredibly short stock piece of wire. So bottom line to that is not modify any certified units at all and instead use kits or your own built FM transmitters and do everything possible to maintain it’s field strength to the rule, no matter what kind of antenna you decide to run or how much coax you use or how high up the thing is in the air.
Then we get to the “Big Numbers”..all behind the decimal point, which really makes them not so big, even when comparing mW to uW, does not matter one is ONE BILLIONTH more than the other, both are still behind 1 and the decimal point which makes both INCREDIBLY WEAK. At that point, the Big Number Billion more times than the other means just that…a very LITTLE billion.
Power level is not the issue nor the rule. Field strength is. And field strength is NOT measured in milliwatts or microwatts or even watts. It is measured in uV/m, mV/m, V/m, none of which is any TPO or transmitter power output measurement.
RFB
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