This is my first post, and I would like to mention that I am a total noob. With that in mind please be kind in your comments. I am glad this is here because I want to start a station for my youngest son and I. He has a learning disability and finds it difficult to communicate. After mentioning the station to him, the first thing he did was right a report on Robin Wiliams and depression. I am will to do anything to see this continue.
I am having issues tuning the antenna.
Starting from transmitter, I used 40 foot of Belden 9913 RG8/U to a Comet Antenna. I estimated the top length to be 20.75 or so for 101.5. I could not find the equation, so I guessed as close as I could. The transmitter is a cheap low mwatt China made. I bought that as a test befor spending hundreds on a FCC cert transmitter.
When I try to tune it the best I can get is SWR of 2. Instead of bring down the antenna to adjust it. I went up and down the rf to see if I was even close. It seems like I am, but it will at times jump up to a SWR of 5. Now before you scream and yell at me, I need to say there are only 3 rf that are taken where I live, and they are low power with the closeest one that is using 88.1 which is 60 air miles away. That might be hard to believe but totally true. I want to say, but can't remember the total number of free rf, it is saved of main PC, but I want to say it is soming like 63 totally free rf. When I went up and down the rf to see if I was close, I at no time came close to any other station. i don't think it would matter anyway. I did a test run and didn't get even down the block.
My question is this.
could the cheap transmitter have a direct impact on SWR? The pl-259 connectors were professional soldiered. Would it matter that it is not grounded yet? I am at this point, when I can, testing things. For the most part, I am having fun with father son time
If the cheap transmitter is the issue, it is not a issue of buying a new transmitter, but I would like to know how to fix it before I do so.
please respond in noob terms. I might be able to save your life in the ER, but this is a totally new hobby. Also pls forgive any misspelled words. This IPads auto correct is not automatically correct.
thx for your time and the forum.
First, welcome to the forum and I hope you find helpful information here.
I searched for Comet antenna and found a couple of models so I am not sure which you have but if it is a quarter wave ground plane type then the length is 2808/frequency (MHz) inches. 101.5 MHz would be 2808/101.5 = 27.7 inches. This is probably about 5% long but it will be a good starting point and you can shorten it by 1/4 inch at a time and watch the SWR.
Anything below 2:1 is OK for this application. You asked if the transmitter could prevent 1:1 and one answer which comes to mind is if the transmitter was producing spurs or harmonics this could cause the SWR to be high. Spurs and harmonics are frequencies which are not wanted and not equal to the carrier frequency. These signals, since they are not at the antenna "tuned frequency", would add to the SWR reading.
What method or meter are you using to measure the SWR?
Be aware that if the transmitter output power is enough to drive a SWR meter then it is possible that the signal strength will exceed the legal limit in the US. Ask if you need more information on this.
Neil
Thank You for the reply. I did forget to post which Comet. http://www.cometantenna.com/ncg/wp-content/uploads/2013/09/cfm-95sl_spec_sheet1.pdf
If this is the one you are talking about then I am way off on the length. However their install sheet mention the length for 103 is 20" and 100.5 is 21.5.
i was and am worried about the harmonics and spurs, so I bought a low pass filter juSt incase. I am buying a dummy load to test the transmitter, which I should have done in the first place.
If this is the one you are talking about then I am way off on the length. However their install sheet mention the length for 103 is 20" and 100.5 is 21.5.
The link posted for the Comet CFM-95SL shows that its length varies between 5'6" and 7'8" (depending on the frequency). Such lengths, and the 3.4 dBi gain for it shown in the link suggest that this is a half-wave antenna.
A half wavelength in free space for a frequency of 101.5 MHz is about 58 inches, which is shorter than the minimum dimension given in the link. But that dimension might include the short section below the horizontal wires, at the bottom of the antenna.
If the installation sheet you have for your Comet antenna shows 21.5" for the antenna length for 101.5 MHz then possibly you have the wrong sheet, or maybe a Comet antenna other than the CFM-95SL. It would be good to clarify that with the antenna supplier before changing anything on the antenna.
Also - you might want to verify that the input impedance of the antenna is 50 ohms, which would match the impedance of the coax cable you are using to feed it. The natural input impedance of an end-fed half wave antenna is quite a bit higher than 50 ohms, so for low SWR some kind of network would be needed at/in the antenna to match it to 50 ohm coax.
You mentioned using a cheap transmitter made in China. Typically those transmitters produce far, far more power than is needed to meet FCC regulations. For example, a dipole FM antenna with a gain of 2.15 dBi and radiating just 1 milliwatt produces a signal that is about 280 times stronger than legally allowed by the FCC. This may not be of concern to you and your son if you are not located where FCC rules apply.
Hope your FM startup and operation goes smoothly for you.
I have used with a high degree of success the Comet Antenna. The SWR can be trimmed to 1:1 with no problems. So, if you are having issues with SWR the first thing I would do is locate a ham radio operator who has a SWR analyzer like a MJF analyzer. That way you can take the transmitter out of the circuit.
The antenna is a 5/8 wave antenna and indeed presents a 50 ohm load due to a shunt coil at the base of the antenna.
I would place the anayzer on the transmitter end of the coax and wiggle the connectors at both ends to see if the SWR changes. I suspect the coax (regardless of who put the connectors on) as the problem. This is predicated on the fact that you cannot get the SWR down to 1:1.
You can use the SWR meter but it just makes it more cumbersome since you are dealing with the coax between the meter and the transmitter and between the meter and the antenna.
Let me know more. I can help.
Encouraging the desire to write is the most important thing about this story, and writing for radio increases the enjoyment.
Writing does wonders to increase thinking skills, so be sure to have stacks of blank notebooks handy with pencils, pencil sharpener and good lighting.
When the radio microphone turns on these written pages can be read aloud for listeners at their radio, which trains good speech ability and thinking can improve also because the reader might think of things to add which are not written down.
Very bad reviews have been written about the Chinese FM transmitters for the reasons already mentioned, including too much power for legal U.S., and very bad spurious out-of-band harmonics.
But when set for a legal U.S. part 15 power, your signal should go about "down the block," but no farther.
If you want to send farther into other blocks and neighborhoods the answer is to use an AM transmitter, here is an outstanding source:
Both the AMT3000 and AMT5000 are excellent.
They are kits, but if you can save my life in ER you can build a transmitter and your son should find the process interesting.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be."
Lord Kelvin
Let's keep with the topic of the original post which is to help the OP with his antenna setup/troubleshooting.
General comments about and reviews of transmitters can be posted in a new thread.
Thanks
Neil
Thank you Radio8z.
wdcx You might be right about the cable. I don't believe it is the cable running from the low pass filter to the antenna, but the pig tail that I made from parts that I bought from Radio Shack. I live so far from anything that I have been trying to order one but this transmitter has a TNC connect and everything else is PL-259 which after several attempts I now understand there is a difference between N-type and pl-259. I have made the 2 hour and 45 min drive into town a few times. There is no HAM store. On a side note, it would have been nice if I could have continued to study this subject in college instead of dropping out to take care of my wife who almost died while giving birth to my son. Nursing was less than two years and is recession proof.
The cable that came in today has TNC male to a N-type male, but I needed a tnc male to PL-259. This cable is using LMR 400 cable, so I ordered a n_type to pl-259 connect and should have it in by Thursday.
As far as AM, I have been contemplating it, however, I have a strong desire or obsession to see this SWR crap to the end. Then after seeing a clean signal, it does not bother me ebaying everything off and going AM. It would be nice to work through this issue, so I can understand what and why I am having this problem instead of going into a panic after finding out the setup I have might not be legal. Hell there are a handful of houses around me and a 1/4 mile to the next house to the east. a half mile to the north, and 10 miles to the west and south. If the FCC breaks in my door over this, I will gladly give them everything. Before that happens, I am finding out why in the hell the SWR won't go 1:1 or close. I admit that I know nothing about this, but I am figuring this out and my son is reading his report do, die or a $1000000000 fine.
If you only knew how hard it is to live here and do anything, but I love the people around me. They are poor farmers and many of them don't have running water or electricity. You might at this point say BS, however, I wish that it was... I understand now AM is the way that I should have gone, however, this whole thing started with me seeing a transmitter on Amazon....
MFJ analyzer was mentioned. Thank you. Would a MFJ 247 work because the MFJ 259c is running $269?
I thank you for the help and for listening to me vent.
If someone could suggest a beginner book to read that explains things like "The Low Power AM Broadcasters Handbook". A book for FM for now would be great since I am working with FM at the moment. I probably will go with AM later due to the hilly terrain. The carrier current sound interesting even though it might not work. I never knew you could transmit that way.
A few years ago I bought a house that needed the electric wiring replaced. I bought a college book explaining everything down to drawing out exactly what needed to be done. Everything is now up to code and passed inspection. Saved me $7000. I just last week gave the house to my brother because his family lost everything. I say this to acknowledge that I might be able to build a AM transmitter, however, one step at a time is a good thing.
I have so many questions, however, I need to figure this out in order to maintain my sanity. Here is a health question for you. Can you develop OCD while learning how to setup a how to start a radio station? LOL
Here is a Comet CFM-95SL Manual Link http://www.truckerswereld.nl/uploads/pagetree/files/Comet%20CFM95SL%20manual.pdf
I have the antenna set to 20.75 on 101.5. Is there an equation to use to find out the exact or approximate height?
This formula, supplied by Wholehouse 2.0 FM certified transmitter for international users, provides a simple formula for making a full wave antenna for the FM band.
On a calculator divide 11811 by the frequency in MHz, and this will give the length in inches.
My math quote was humorously directed at 1 Carl Blare.
His antenna is 5/8 wave.
.. linked in Reply #9 above shows the length of the top section of the radiator for different frequency spans. That top length is adjustable. The top length plus the fixed length bottom section produce the radiating length needed for the antenna.
Standard equations to calculate wavelengths in free space do not apply to this situation. The manufacturer's installation guide should be followed, instead.
Rich was I right making the length 20.75"? I asked Rick at Comet for an equation and he emailed me the manual. LoL I should have asked him the length for 101.5.
I found out that a lot of times these cheap transmitters will when connected to an antenna, that is not provided by them, give weird SWR readings and not be able to be tuned. When A dummy load is connected, because of the resistance, it might give a reading of 1:1, but when connected to a better antenna this could happen.
Who knew when I pressed the buy button that I should have bought a Ramsey or Rangemaster. Well the new pl-259 connect comes in tonight and a dummy load Monday, and this sorry transmitter gets shipped back after I can proof it is defective.
It will be refreshing to be able to start over knowing what I know now.
Well it wasn't the pig tail like I was hoping it would have been. So I bought a MFJ-259c from Amazon. This should tell me not only the tru SWR but the frequency the antenna is set to. I can send back the diamond. This should let me know if it is the antenna or cable or both. Thanks for the turn on to this. Really thank you.