Ramsey TM-100 antenna

The TM-100 is a very usuable antenna. But, you are wise not to use it in the location you mention.

I have a Ramsey FM-30B that I built. Full ouput for the kit is about 35 mw. At 1/3 full output on the rat-tail antenna, its signal gets out about 2 city blocks from a second story bedroom; the transmitter and antenna are sitting on an equipment rack in the studio, indoors.

So, with the TM-100 and a FM25 at full output on top of a hill.....uhhhhh, no. The FCC recommends a range of approx. 200 feet; or about what my station does. As far as the FSM, forget it. just turn down the transmitter output until you cover your 200 hundred feet and forget it. This shouldn't be brain surgery.

Rev. Marshall Johnson, Sr.
Senior Pastor, President
Rhema Christian Fellowship, Inc.

Rhema Radio - The Word In Worship
AM 1660 - FM 93.5
http://www.rhemaradio.org

Ramsey support did get back to me this morning. They suggest it will be fine as long as I use less than 10 feet of combined antenna and coax feedline. While I understand that it is still entirely up to me to make sure my output falls within the rules, having them say that it will meet the specs. helps in the event of a problem. I did run my FM25 from a simple dipole on a short length of coax (4.5 foot dipole and 5 feet coax), and it was pretty meager. I asked for a little more info regarding this just to make sure that mounting the transmitter a foot or two from the antenna would be "legal", so I'm now just waiting for a reply (and the antenna). In my application, running a long length of balanced audio and DC power would be easier than running new 75 ohm coax through the building and up the tower. Worst case I'll turn the power down, or put a pad on the coax.

Hi,

I don't understand why Ramsey or anyone else would say you would be fine if you use less than 10 feet combined antenna and coax using a FM25 FM transmitter. Are you sure that you are quoting Ramsey correctly and that they understood that you were asking about FM? The restrictions on antenna and feedline are for AM, not FM. FM is regulated by field strength.

I will advise that you avoid FM if you intend to broadcast. The rules restrict the field strength on FM which is very different from the rules for AM. If your FM signal can be heard beyond about 200 feet you are exposed to problems.

Neil

Yup, that's right. They still suggest keeping the antenna+feedline length below 10 feet. Even though I know this doesn't apply to FM, I'll probably do it since it actually may be easier to run audio (balanced) and DC power up to the tower and locate everything up there.

I got another response that says to stay in compliance I should "turn the power down a little". Great, but what is a little. Give me a voltage reading or something to shoot for... I also asked if it would be good enough to just put a pad on the antenna connection. It's pretty easy to get RF pads for cable TV in 5db increments, I probably have several kicking around in the shop.

And I realize that the burden of staying legal ultimately falls on the end user, but they really could help us out here! One of the local broadcasters gave me info on renting a FSM, I might have to check into that and see what it would cost for something that can measure down to peanut power. Also still trying to convince my boss that AM will have better reach, and is easier to keep legal since there are things that we can measure.

The SStran transmitter shows 20-20K response, that should sound at least as good as the mono feed going to the TV modulator on the cable system going through the dorms. Might even sound better since it may not be going through a low-fi TV speaker

Hi Greg,

I guess Ramsey has a reason for their recommendation though I don't know how this applies to FM unless there is some technical advantage to limiting the length that I don't see.

Lacking a calibrated FSM, the only way I can suggest to get near compliance is to adjust the power out if there is such an adjustment or to add atten. pads in the antenna line to get a range of about 200 feet. Specifing a voltage or power won't work unless the antenna gain is known and the power can be correlated with the field strength. Others have calculated that 11 nanowatts applied to a center fed dipole will produce the legal field strength but this low power would be difficult to measure without $$$ equipment.

As an aside, let's assume the transmitter is adjusted to an output power of 10 milliwatts and you want to pad it down to 11 nanowatts. The attenuation will need to be 10 log (10 mW./11nW.) = 59.6 dB. or about 12 of the 5 dB pads in cascade. That is just a ballpark number to see what is involved if you are using an efficient antenna.

Another approach is to use a very short antenna which is cut to give the 200 foot range. That is what I did with my FM-25A which is mounted in a metal box with the whip antenna vertical through a hole in the top of the box. 4" of antenna extending above the box gives me around 200 foot range with a car receiver. The short antenna reduces the signal due to the mismatch with the FM - 25A transmitter and I have never had any problems with this damaging the transmitter. I have even run it with no antenna. The FM-25B or other transmitters may not be as forgiving as the FM-25A and a pad in the line would provide some protection from reflected power..

You are correct about AM being easier to set up in terms of compliance since the wire lengths and input power are very easy to measure. Many here have reported good ranges by using a base coil loaded 3 m. antenna and radial ground system.

I have been using the SSTRAN AMT3000 here for over a year and am pleased with the audio. The internal audio compression system allows almost a set and forget system.

I hope some of my comments are useful to you.

Neil

I guess Ramsey has a reason for their recommendation though I don't know how this applies to FM unless there is some technical advantage to limiting the length that I don't see. Lacking a calibrated FSM, the only way I can suggest to get near compliance is to adjust the power out if there is such an adjustment or to add atten. pads in the antenna line to get a range of about 200 feet. Specifing a voltage or power won't work unless the antenna gain is known and the power can be correlated with the field strength. Others have calculated that 11 nanowatts applied to a center fed dipole will produce the legal field strength but this low power would be difficult to measure without $$$ equipment.

The FCC gives the equations to calculate Part 15 FM field strength on page 32 of their OET Bulletin 63, which is available for download on the FCC website. Its title is UNDERSTANDING THE FCC REGULATIONS FOR LOW POWER, NON-LICENSED TRANSMITTERS. Of course the equations are also found in standard antenna engineering textbooks.

The equations the FCC published apply to an isotropic radiator, which doesn't really exist in the real world. So the power that those equations calculate needs to be adjusted for the real gain of the antenna used.

A 1/2-wave dipole has a peak gain 1.64 X greater than an isotropic radiator. So if a 1/2-wave dipole is used, the radiated power needed for the maximum Part 15 field at 3 meters drops to ~11.4 nW, instead of the ~18.75 nW calculated with the FCC equation (18.75/1.64).

Measuring either Part 15 power or field accurately takes some good equipment, and measuring the field strength needs a "qualified" test site. That's why it is better to buy a Part 15 FM tx with an integrated antenna that have been measured together as a system, and received FCC certification. Then if it is used unmodified, you can be fairly certain that you are in compliance.
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I guess the part that bothers me most is that Ramsey says in the instruction manual (page 4 I think) for the TM100 that is is the answer to staying legal with your FM25/30 transmitters, while gaining some (unspecified) range. If they had come right out and said that you needed to run at a lower power by doing X to the FM25 or running the FM30 at Y percentage, I would be happy.

Anyone familiar with the FM25 circuit? It looks like changing the value of R19 (near jumper 2) will change the output power. Jumper 2 is used in non US markets, and a 2.2K ohm resistor in US markets. So it would appear that replacing R19 with a higher resistance would do the job, but finding that resistance will be the harder task. The other choice might be to lower the supply voltage down to 8 or 9 volts, which should also lower the output power.

For the older FM25A support suggests using a 2.7K resistor in place of R19. Apparently the older FM25A had a bit less power than the newer FM25B and FM30.

Hi Greg,

I assume you are looking that the schematic for the Ramsey FM-25A. I just looked and it appears that R19 does establish the base drive for Q2 and perhaps affects the output power. It sure would not hurt experiment by changing this R but I think a better approach would be to attenuate the output after L3 at the antenna connection. My reason for this is that an amplifier can be "starved" for an input signal and could produce distortion, though this appears to be a Class A amplifier and it may not matter what the signal input is. Though this tx. has a LP filter on the output, it may not be sufficient to supress harmonics unless driven with the signal strength other than for which it is designed. Try it and let us know what you find.

By the way, is anyone else seeing red when viewing the posts text. I find it very annoying but maybe it is just my browser being wierd.

Neil

I only see red in the headings of each post.

I'll let you know what happens when I change that resistor. I want to see what the range difference will be between 2.2K and 2.7K. Mearly curiousity, I'll run it with the 2.7 unless it sounds like garbage. I kind of wish they had told me to put an XXdb pad on the output, seems like it would have been easier.

It's an ugly plastic thing, feels kind of weak but I haven't glued all the joints yet. I did do a test with a cheapo radio, and without decreasing the power it seems to go about 200 feet (just like it should). With a better receiver you might get better coverage. Test conditions were, cool and not raining. Antenna center was about 8 feet off the ground, held up by gray PVC pipe, about 10 feet away from the building. About 15-20 feet of known good RG6 between the transmitter and antenna.

It was a simple kit to build though they could have used some different sized PVC for the mount portion to make it stronger.

I see an SStran AMT3000 AM kit in my future with the antenna that they have on their web site.

Greg,

Thanks for the report on your antenna experience. I don't know how much time you have to try things but here is a thread from another board where a homebrew J pole was discussed. I thought you might be interested.

http://www.ramseyelectronics.com/cgi-bin/ultimate/ultimatebb.cgi?ubb=get_topic;f=1;t=001689

Just a note that if you go to AM with a base loaded antenna, the tuning is much more critical than for the FM antennae but the greater range achieved with a legal setup over FM is worth the effort.

By the way, the "red print" problem is resolved.

Neil

I actually had better range with a dipole made out of a couple old light stands connected to a long piece of RG58. I assume that there was a mismatch and I was radiating from the shield.

I'm not really fond of the way that one portion of the TM100 goes together, so I'll probably pull that apart and make sure it is right. Where you solder the coax to the circuit board it can get a little weak if heat is applied for too long. So I think I'm going to trim that back and try it again. That is the only place where there could be any failure, the rest is just too simple to have made a mistake. They also give you an extra mile of coax that needs to be delt with, maybe I made some kind of problem when I stuffed it all inside the pipes. I'll let it hang out of the bottom for the next test. And finally, they should have used a larger diameter pipe for the mounting portion. They could have easily used a reducing T for the radiator part. That would give a lot more strength to the design, and probably make it easier to clamp to just about any type of mast.

I hope you didn't take the whole length of coax and stuff it inside the "L" shaped portion of the antenna. If so, shorten it (as per the directions) and put it back together. If the RG-59, 75 ohm coax is the right length, the feedpoint inpedance is very close to 50 ohms. At least, it has been on the antennas I have constructed. The antenna also should come with a BNC to F connector adaptor. If your kit didn't have one, call Ramsey and ask them to send you one. Then you can feed the antenna with RG-8X or RG-58.

And finally when you get the antenna constructed the way you like it, paint it with flat black latex paint. It will shed ice in the winter, block the destructive UV radiation to the PVC pipe and look a lot nicer. As far as the bigger pipe is concerned, it would only add considerable weight to the antenna. Hopefully you glued the antenna together, right?

Marshall Johnson, Sr.
Senior Pastor, President
Rhema Christian Fellowship, Inc.

Rhema Radio - The Word In Worship
AM 1660 - FM 93.5
http://www.rhemaradio.org