I just added a dipole antenna outside of the mobile home mounted horizontaly on my deck. After some disappointing results on coverage, i could see i wasnt anywhere near 200 feet (range wise) with the antenna inside the mobile home.
Too much reflection and resistance caused by wiring and the aluminum siding. I wonder though, what kind of gain a dipole antenna would have if its cut for 95.7 mhz. I am using a 75ohm to 300ohm balun, the kind you get with a tv/fm antenna.
(Last I knew a dipole like this had little to no gain.)
The wire is 12 gauge automotive wire.
Cable is 75 ohm cable/tv coax.
And I feel it should have a attenuator inline as well just to be safe.
I hear no intereference on any other parts of the fm dial or in any other bands for that matter.
So how efficient will this be? By the way, the antenna is probably 5 feet off the ground.
Any input would be great.
thanks
Why horizontal?
Horizontal dipoles are somewhat directional. So the signal will be stronger at 90 degrees to the antenna than to a receiver sitting in the direction the ends point. Is that intentional in your setup for some reason?
I don’t recall what transmitter you run, but with a dipole cut/adjusted to anything like 1/4 or 1/2 wavelength, you’ll almost certainly need to attenuate it to be even close to part 15 regs for FM.
The amount of power allowed to radiate is very tiny, and it doesn’t take much of an antenna to do the job for FM. But on FM, a little goes quite a ways.
For comparison, I run an FCC approved FM part15 transmitter with the supplied antenna. It gets a good solid signal to a cheap portable radio at about 150 ft and can get just under 2/10ths of a mile to a reasonably good car stereo. Not talking a “fringe” signal, that’s with a nice listenable signal that stays in stereo and with no fadeouts until almost 2/10ths of a mile ( .18 mile to be more precise). I don’t live in a trailer park either, but I *do* live in the residential section of a small city, so it’s certainly not optimal either. I’ve tested a few different commercial FM transmitters that have FCC ID numbers and that seems to be fairly typical.
You can see a pic of it here:
Ok, see that bitty piece of black wire? That’s the antenna. All stock, nothing modified, just as it came and just like when they tested them for FCC approval. That antenna is in the FCC specs for this transmitter, I think the FCC site said it’s about 80 *millimeters* long. Less than 4 inches.
Now, I run on the low frequency end of the FM band, so a resonant dipole would be about 5 and a quarter *feet*. That’s about 20 times the size of the antenna on this particular FCC approved transmitter.
So that’s my logic behind saying that running an antenna of the size you’re talking about would definitely need attenuation to be in the legal 250 uV at 3 meters allowed. I think Rich calculated the power needed with a dipole sort of like you’re using being about 11 nanowatts to be at the 250uV limit? I don’t recall what sort of transmitter you have, but it’s almost certainly more than that.
The problem is that there’s almost no way for a hobbyist with typical modest resources to measure and see if they’re in compliance for the 250 uV at 3 meters limit for legal part15 FM.
Daniel
Balun
The 72 to 300 ohm balun (1:4) is for use with a folded dipole or with 300 ohm TV twin lead, not coax.
For a simple dipole you need a 1 to 1 balun for best match if you are using coax.
Since you may not need to radiate much power to stay legal, the choice of which balun to use may not matter. The mismatch with the 70 to 300 ohm balun may reduce your radiated signal.
Neil
That was my intention ..
I knew there would be loss using the 75 ohm to 300 ohm balun, I really dont believe the transmitter i am using is all that legal ( Ramsey 25B ).
Although i did have a technician do some work to lower the voltage, i saw a signal of 1 and 1/2 miles using the dipole i mentioned!!
This suprised me and then had me worried at the same time.
I did however check the voltage again and i must have raised it when i was running the telescopic whip that ramsey supplies with the kit.
The transmitter was operating inside the mobile home using the whip antenna. I knew this was going to hurt me and it did.
While using the indoor antenna i was seeing or rather hearing my signal fade and chop at 70 some feet using both a portable radio and a car radio as a test. Metal and house wiring, not to mention the other mobile homes caused so much reflection and multipath my station would come and go. It really was frustrating.
You can imagine my suprise when i drove a mile away from home to pick up something from the store, my signal was strong in some places and gone in others, but when i reached the 1 and 1/2 mark i quickly got what i needed from the store and came straight back home.
I have lowered the voltage and the station is audible at 100 to 150 feet. At this point my wife and i are talking about going AM and doing away with the questionable FM transmitter. I would rather cover a few miles legally than to have a letter in the mail or a unfriendly knock on the door. 🙂
Right now its just a matter of monetary funds, but i have my eye on the Procaster, it would be perfect for the limited space i have to work with here.
But for now the Fm seems under control, im not covering the whole trailor park, but i do offer service to one end of the park and im happy with that..
Edit: I should have mentioned the specs for the Ramsey fm25b on RF output are 5uV to 18mW, clearly this transmitter has not been certified by the FCC. So anyone using one for Part 15 should take extreme caution when operating the fm25b or the fm25 both transmitters are capable of higher output than what is allowed by part 15 standards.
My search for a affordable transmitter continues!! stay tuned.
Radio Sunshine
FM dipole, etc.
Rock95seven,
You have a firm grasp of the legal situation with part 15 FM.
You wrote:
should have mentioned the specs for the Ramsey fm25b on RF output are 5uV to 18mW, clearly this transmitter has not been certified by the FCC. So anyone using one for Part 15 should take extreme caution when operating the fm25b or the fm25 both transmitters are capable of higher output than what is allowed by part 15 standards.
I will add a couple of comments for you and others to consider. First, 5uV to 18 mW? Typo?
Second, the limits on FM are based on field strength and not transmitter power. Conditions other than power affect the field strength, such as the type and placement of the antenna. With even microwatts of power out of the transmitter it is possible to exceed the limits by many times. Your approach is the same as mine: adjust the transmitter or antenna so the range is about 200 feet.
In my case, my FM25 tx. is in my basement and the antenna is a 4 inch whip. My signal is barely audible on a car radio just over 100 feet away yet it is loud and clear inside my house. Since I only use this for personal reasons it works very well for me.
Anyway, thanks for your report and good luck getting the AM system of your dreams. Just a comment that a few miles of range may be a bit optimistic.
Neil
I wish the FCC would allow
I wish the FCC would allow FM Broadcasting on Private and Semi Private properties like they do with am on campuses. something like allowing FM within a well defined area where you would need to comply with F/S at the property line. this would allow trailer parks, campgrounds, apt. complexes, theatres, parks, theme parks, campuses, etc. to use better quality FM instead of am. I think canada already has this type of service on the books
Thank You,
Rev. Robert P. Chrysafis
Universal Life Ministries
http://www.ulc.org
Moderator Hunterdonfree
http://groups.yahoo.com/group/hunterdonfree
Ramsey “PVC” pipe FM dipole
The reason for inefficient antenna systems on certified FM transmitters is to assist the manufacturer in designing and building a cost effective product and helping the operator to avoid interference troubles with licensed broadcasters. But, that doesn’t mean an efficient FM antenna system operated with just enough power to be legal doesn’t work and sound better.
I would rather operate a good sounding station at 1 milliwatt or less than one of questionable quality that can be received beyond the acceptable 200 foot range. Turn the output down and put the signal on a matched antenna system. The difference in the quality of the received signal is remarkable. The FM carrier will decrease the amplitude of the AM component on the signal. And that can be accomplished by simply impedance matching the transmitter to the antenna. The stereo signal is substantially improved as well.
I have been a broadcast engineer for decades and have experienced similar results with “super tuned” high power broadcast antenna systems. Try it. You will be amazed.
Ramsey sells a little home-brew PVC dipole antenna system for little cash. It is a dipole. And when matched to the transmitter, produces a high quality FM signal. Just give the antenna enough power to be received at 200 feet or less and you’re in business. Want to experiment? Give this a try.
Rev. Marshall Johnson, Sr.
President & General Manger
Rhema Radio – The Word In Worship
The Dalles, Oregon, USA
http://www.freewebs.com/rhemaradio