Fellow Radio Scientists
Greetings from my FM and AM transmitter room!
I’ve read all the posts and articles on the whys and how’s to connect a balanced antenna to an unbalanced feed line (coax)
I’ve researched matching networks and read up on how to create a balun to match a balanced to unbalanced line.
I’ve tried the round omni directional antenna that MCM sells for FM reception but I used it for transmitting, these have a built in balun I assume for 300 ohms (the antenna is a round folded dipole) to 75 ohm coax. This worked good but thy they have less gain than a dipole since a dipole is best in two directions.
What works the best for my attic FM installations? I ended up using a TV rabbit ears type dipole antenna, mounted running up on the slope of the rafter, for convenience.
I have tried a matching balun with the rabbit ears, 300 ohm to 75 ohm, but a simple dipole has about a 73 ohm impedance so I found best results was to connect the 75 ohm cable directly to the 73 ohm antenna, balanced to unbalanced, I know that is not correct, but that’s the way I did it.
Here is the good part.. by using a field strength meter, I can simply tune the antenna by telescoping in, (from fully extended) the “ground end) of the dipole and watch the field strength climb till you hit resonance, so that is at least what I think is happening.
Now here is my question for the real wizards out there, those who like me are constantly trying to find the answers and the truth… So what am I really doing when moving that antenna rod? When I move the antenna rod in am I reaching resonance, or just creating a big mismatch in the balanced/unbalanced connection causing the coax to radiate which is causing my field strength meter to go high? I moved the meter away from any coax and still get a nice peak in my readings. All I know for sure is the strength of the signal greatly increases as you push the antenna rod in, and then hits a peak. If you go past the peak when tuning the antenna the signal will then decrease. You can move both rods in (from both ends of the dipole) when tuning and reach the same results but it’s easier to just move the grounded side in as the there is less de tuning effect as a person gets near the antenna as you are moving the rod in. Both rods are about 38 or so inches long fully extended, because they are from a VHF TV so there is some length to spare as far as the correct length (33 inches or so depending on how far above ground and things around the antenna) for general FM transmission.
I have a real antenna farm in the attic, I have five of these dipoles up there and they all tune the same way, but each has a different transmitter attached and different frequency, so the rods end up different lengths for different frequencies. Let me know what you think is happening. I like to believe that I am tuning for resonance, the only thing that throws me a bit is I’m wondering if the balance /unbalanced system is tricking me somehow. I once read that it’s more important to match the feed line Z to the antenna Z than be concerned about the balanced/un balanced issue. I feel both are important, just stating what someone else was stating.
Radio Joe
By pushing in you are tuning the antenna to 1/4 wave on each pole. You would gain better results it you push in both sides.
I did a tinfoil test on rabbit ears and my range was boosted. I should try this test with the antennas pushed in and see if I can get better range.
🙂
Hi Joe,
Having recently tuned an outdoor 40 meter dipole this is fresh on my mind. Changing the length of the antenna changes primarily the reactance seen at the feed point. The idea is to adjust the length so the reactance is zero. When this is done the impedance will be purely resistive but may not be the ideal resistance for the coax. By moving the elements into a V pattern the resistance can be adjusted somewhat. For ham power levels this is easy to do my measuring two things, the resonant frequency and the SWR, but this is not practical for Part15 FM power levels so the next best thing is to monitor the field strength as you are doing. You may have to go back and forth with the length/angle adjustments to find the true maximum field strength.
The elements should be the same length otherwise you will probably not get a dipole radiation pattern but this pattern will have two big nulls which is not usually wanted for broadcasting. By adjusting only one leg your pattern will still have nulls but they may not be as deep.
A simple way to eliminate a balun and still prevent feed line radiation is to slip a toroid core over the coax at the antenna. The Amidon T-50-12 or T-75-12 works.
Since you have already done reading on this you probably have encountered the information I posted, but nevertheless I hope this helps.
Another thing to consider is that for Part 15 FM there is a limit on the radiated field strength in any direction. It is possibly not worth the effort to finely tune an antenna if an untuned antenna yields the allowable field strength.
Neil
Good evening Radio Joe, and Neil, stay tuned, might need comments.
Since you mentioned having several transmitting antennas in the attic, I am wondering if you should turn them all off except the one you are tuning.
All the combined energy might get picked up by the antenna you are tuning and cause false readings.
Then when you have one antenna tuned, turn everything on and see if your readings change.
Thanks for all the replys, yes, I only use one antenna/transmitter at a time for testing. And thanks for mentioning ,I was aware of chaning the inductance/capactiance for resonance by adjusting, but it was strange to see it happening when adjusting only one end of the dipole. Im wondring if i put a small varable cap in series with the grounded leg (rod of antenna connected to the shield) and tune that (leaving the lenght the same) would that clear up the issue on the nulls? Ill add a ferrite choke on the end of the coax, heard about doing that also, just dint do it but that is good advice! I didnt know about the nulls, I was just happy to see the tuning work!. I have tried over and over again to make a simple antenna tuning box for these antennas but never get the values right so I thought there must be a simpler way to do this, and I found it. I have tried befor to adjust both ends of the antenna but tht was sort of hard to adjust it up there and then check the meter, as the readings change when your body is in the area. I have also comptued the proper lenght for the frequency and tried setting the rods that way and check strenght, but as we know that sometimes isnt perfect either and it was hard to tell improvements in Signal strenght. So its nice to actually see that meter pin out when making adjustments, makes life worth living!
The resonance for a quality dipole is rather sharp, enough so that within a typical amateur band it is a compromise to keep the tuning operable throughout the band. Broadcasters have an advantage in that they work on a fixed frequency and can optimally tune their antenna accordingly. On the ham bands a SWR of 1.5 : 1 is acceptable. At a local high power FM station here the transmitter shuts down at 1.15 : 1.
Consider using a vertical dipole. The nulls will be up and down rather than horizontal. The inverted V which I think you are using radiates a strong signal upward from horizontal which is great for ham use but will go over the heads of your potential audience. Maybe invert the inverted V?
Typically, an antenna tuner is used to make the transmitter see a proper load despite serious mismatch with the feed line and the antenna. Tuning the antenna with a variable cap or inductor as you mentioned makes sense and will work and is often done for transmit antennas. In the case of a dipole, it is much easier and lower loss to just cut it to the proper length and call it good.
Neil
I would like to have vertical dipoles up in the attic, but the attic is tight, so the dipoles run up at a slant mounted on the rafters, works nice but less than ideal, vertical would be best I understand.
I have a chart I made up which has the proper lengths required of each leg for a half wave (88-108) dipole at the frequencies I have transmitters for. I once had each antenna set up that way but I never fully tested for field strength. I would test this by using the antennas fully extended and then properly adjusted to correct length and document differences in field strenght. I didnt continue testing very long because of the heat in the attic at the time and also because I was getting different readings which didn’t make sense and were not consistent.
So I gave up the project at the time and tried the round Omni directional FM antenna from MCM electronics, not sure if you have seen those, it’s a half wave folded dipole but wrapped in a circle. So how well is that supposed to work? Well connected to my FM receiver it does work as an antenna but signal strength is better in two directions using a simple dipole. The antenna has merit for the price of ten dollars. It were roof top mounted it would provide good local reception I’m sure, or even good local transmission for that matter.
As we know, using a horizontal dipole for my hobby broadcasting doesn’t work out very well because of the horizontal polarization. This is because for receiving, automotive antennas are vertical (The car radio is my primary source of testing for range). So when testing range a person is already at a disadvantage because of the horizontal/vertical problem of coupling.
With that issue understood, getting the capacitive reactance and inductive reactance canceled out so you have nothing but resistance left in the antenna is enough to make me happy for now!
My primary concern is the antenna system at each frequency is operating at peak efficiency, in covering my home and yard so the horizontal element isn’t a primary concern right now for me, even if range is compromised because of it.
Because of the incline of the rafters where the antennas are mounted, I am getting some vertical radiation from the antennas, but I know if I could mount the transmitting antennas 100% vertical, I would most likely at least double the range or more. I suppose I can cut a 1/8 inch hole in my roof and stick the tips of the dipoles through it so I can go full vertical then make some kind of rain bonnet up there to keep the water out... Ok well ill save that venture for next time!
Joe,
In theory a vertical dipole mounted high would have a null beneath it where part of your house is located causing a drop out zone. In practice there would probably be effects from reflections and coax radiation which makes this unpredictable. If you are only trying to cover your house then a horizontal dipole makes sense since the nulls would be directed outside your house.
My transmitter is located in the basement and is operated only for my wife to be able to listen to music on her portable around the house. The vertical whip is adjusted so the signal doesn't go more than 100 feet outside. Adjusted this way the signal doesn't reach the second floor, perhaps because of an upward null. I did not want a horizontal dipole because of space limitations.
To fill this null, I used some abandoned coax which runs from the basement to the second floor (formerly for cable TV which we dumped) to feed a second antenna which is connected in parallel with the whip output of the transmitter and the transmitter signal ground. At the end of the coax I attached a short wire (about 4 inches) and placed a T-50-12 toroid to choke off the RF on the outside of the coax. This was placed so the coax length before the toroid away from the antenna is the same length as the antenna wire with the intent of forming a vertical dipole. This works well in providing a signal to the second floor.
I changed the length of the whip and this improvised antenna so the outside signal range is still about 100 feet. Such a dual antenna setup will produce nulls and I have observed this but all that need be done is to move the receiver a bit until the signal is good.
My antennas are far from resonant and my transmitter doesn't care. They were adjusted to limit the range, not to optimize the match. This is not what you are doing but I thought you would find this interesting anyway.
Neil
Hi Neil, all information is interesting and valuable when it comes to our hobby interests. and always apriciate any information. Up in the attic there is a lot of things that will do who knows what to the radiaton pattern! I dont have any problems with the signal where I want to receive it right now. I also have two horizantal wire AM antenns up there with loading coils for my talking house transmitters. I very seldom use it all at once, my lights in the house might be modulated with the music if I did who knows. By the way is there any way I can post pictures here with out going to some place like photo bucket and getting a link from my account there?
Joe
Joe asked "By the way is there any way I can post pictures here with out going to some place like photo bucket and getting a link from my account there? "
You can attach pics on the first post in a thread. After that you need to link from a hosting site. I don't know why it is set up this way but it is.
Neil