I am operating an AM25 on 650 khz. All of the free frequencies here are in the lower part of the band, and 650 is about the best of them, so using a 1500 khz plus frequency is out of the question. Most of the Part 15 antenna I see here on this site and on other sites are for the higher frequencies, with one for the longwave band. I understand there are differences in efficiency between the lower and higher frequencies, but I am willing to live with it. So, without getting too complicated, can someone help me figure out how to construct a properly loaded antenna for a lower frequency like 650 khz? Currently, I am using a 3 meter long piece of pipe sitting right on top of a ground stake, with the lead-ins attached appropriately. I tried the Ramsey antenna suggestions in their manual, and it was even worse than the current set-up. Wouldn't the basic models for the higher frequency antennas would work for the lower frequencies if the electronic values for the components were properly substituted?
I am operating an AM25 on 650 khz. All of the free frequencies here are in the lower part of the band, and 650 is about the best of them, so using a 1500 khz plus frequency is out of the question. Most of the Part 15 antenna I see here on this site and on other sites are for the higher frequencies, with one for the longwave band. I understand there are differences in efficiency between the lower and higher frequencies, but I am willing to live with it. So, without getting too complicated, can someone help me figure out how to construct a properly loaded antenna for a lower frequency like 650 khz? Currently, I am using a 3 meter long piece of pipe sitting right on top of a ground stake, with the lead-ins attached appropriately. I tried the Ramsey antenna suggestions in their manual, and it was even worse than the current set-up. Wouldn't the basic models for the higher frequency antennas would work for the lower frequencies if the electronic values for the components were properly substituted?
PhilcoBill AKA Bill Harms
Elkridge, Maryland
Old Time Radio on 650 Khz
Hi Bill,
I have used both the AM25 and the SSTRAN AMT3000. The base loaded antenna designed for the SSTRAN should tune down to 650 with a bigger inductance coil.
The output network on the AM25 is radically different from the SSTRAN and I don't know if this antenna will work with the AM25. The AM25 appears to be a low output impedance unit and will probably drive a tapped loading coil with the "cold" end of the coil connected to ground rather than floating in the SSTRAN design. It will function as a transformer as well as a loading coil. I am feeding a base loaded antenna here where the AM25 output is coupled to the loading coil via a transformer winding and get a good field strength. I gave a detailed account of this in my post here a few days ago. Check out "AM25 Tech Notes" on this site.
**edit** I am operating around 1500 kHz. but my antenna scheme should frequency scale down. You will have a larger reactance to deal with but it should tune out.
I hope this gives you a start with this.
Neil
Bill,
As a followup, here is a link to an article about antenna and loading coil design. In the article, the author gives some different designs. Maybe this will help you or others with their designs.
http://lpam.info/index.php?page=antennas
Neil
Thanks Neil:
As a recap, it looks like I can use the plans for the higher frequencies except I will need a larger inductor (read more winds on the coil). I also need to pay more attention to the ground plane. This will keep me busy for the next while.
Of interest, just for grins, I hooked up the center tap of the antenna lead directly to the ground rod, and the signal was fairly strong for some distance. In fact, I could hear it about 1/2 mile away in my car when driving near a power line. If I drove down a side street away from the power line, the signal completely disappeared. It seems I was coupling into the power line some how. The audio was okay, except for a constant 60 Hz hum.
I have a couple of questions, is the impedence of the antenna lead out of the AM25 necessarily 50 ohms? Does the total length of the antenna (the three meters length in Part 15) include the antenna lead-in if it is shielded? One could argue that the shielded part of the lead-in is still part of the ground on the chassis because it is really not effectively radiating RF.
PhilcoBill
Old Time Radio
650 KHz
Elkridge, Maryland
Bill,
Power line coupling does occur, but I suspect that your signal "got out" due to the radiation from the ground lead. The hum could be due to the effect on the transmitter where the output sees a RF ground but the circuit ground is floating or grounded through the audio source producing a ground loop. This is just speculation on my part.
I don't know the output impedance of the AM25. The circuit has a step up transformer at the drain of the output FET so the designers apparently decided to raise the impedance at this point. This is followed by a lowpass filter which can further change the impedance. I haven't simulated or measured this circuit impedance.
Transmission lines, even though shielded and grounded to the transmitter, will radiate a signal and that is why part 15 rules limit the total length of the ground, antenna, and transmission line. It is possible to use a balanced transmission line with appropriate baluns which does not radiate, but the rules don't make any exceptions for this That is why most part 15'er mount their transmitters at the base of the antenna. Doing so makes the transmission and ground lengths minimal so the antenna can approach the 3 meter limit on the total length.
Neil
Hello,
Considering you have a 3/4 inch copper pipe of 3-meter antenna and you want to broadcast to 650 kHz, you'll have to build a 2 inches diameter loading coil and make 406 turns of copper wire (24 AWG).
I've calculated it for you based from BOTLOAD2.EXE application (Dos version - still works in Windows) which can be downloaded at the following address:
http://www.btinternet.com/~g4fgq.regp/botload2.exe
Good luck with your part 15 radio project.
Yves Roy
Hello Neil,
I have an AM25 transmitter too.
Once you'll know the exact output impedance from AM25, let me know. I plan to take several days of vacation in the coming months for building up my 3-meter antenna with my own loading coil and burying about 14 radials. Maybe i will try to make some experiences to elevating my 3-meter antenna including my 14 radials.
Regards!
Yves
Ramsey's tech people once told me the output impedence of the AM25 was 50 ohms, as is, I believe, the SSTRAN also. JimB
I have simulated the SSTRAN output stage on Electronics Workbench and found using the variable resistance half voltage method that the Z is about 1200 ohms without R18 installed. I made no attempt to simulate the reactive component.
I will share the simulation for the AM25 when I do it. I would not be surprised if it is 50 ohms, I just do not know now.
Neil
I just wound a loading coil for my transmitter. I am not sure of the performance yet. I have a field strength meter on order and am waiting for its arrival. For grins, I used 24 AWG enamel copper wire and wound a total of 500 turns with taps. The resistance at about 400 turns is about 7 ohms. Is that too much? Should I try it out and see how it performs? Should I used something like AWG 20 or less?
PhilcoBill
Old Time Radio
650 KHz
Elkridge, Maryland
by MRAM 1500 kHz
Speaking of loading coils, I also wound one to experiment with (re-invent the wheel.)
Depending upon your power output, make sure to have adequate space between turns of that enameled wire or they will arc.
Testing mine on the bench, I purposely ran the power up to about 3 watts to see what kind of voltages developed. The cold end of the coil returned to ground, the hot transmitter lead was tapped up about 3 to 5 turns from the cold end and the 8 foot antenna wire was moved to various taps for best field strength. It ended up about 50 or so turns from the cold end.
The screw driver with the neon lamp voltage detector lit up quite nicely and those usually take around 70 to 90 volts just to glow. Putting it at the top end of the coil really lit it up! Just like the old Tesla Coils. And yes, it drew an arc, tiny as it was, so the voltage must have been up there. Of course, you probably don't have to worry as your transmitter output is probably less than 60 or 70 milliwatts.
Oh, don't check for that with your finger. RF burns smell funky.
I thought this was interesting:
"(the) 160 meter coil... is wound with 63 turns of 12 gauge wire at 6 turns-per-inch. This coil was designed primarily for 160 meter operation and will operate quite satisfactorily on 75/80 meters but tuning on the higher bands is not recommended because of the excessive shorted turns. A whip at least 7-1/2 feet long with a 20 inch diameter capacity hat is required to load this coil on the low end of 160 meters. If a whip at least 8-1/2 feet long is used, the 16-inch capacity hat may be used with this coil for 160 meter operation. For portable operation, a 10-foot whip, made up of a 102-inch whip and an 18- or 24-inch extender mast, may also be used to give full 160 meter coverage without a capacity hat. Adding a 20-inch capacity hat to this combination will extend operation down into the 1700 kHz experimenters band."
Empasis mine
These coils seem wound to exacting specifications, especially with regard to turn spacing and wire diameter.
I wonder if we could learn something from this?
These photos linked from this page:
www.texasbugcatcher.com/cata/tbcspec.htm
Hello,
In order to get frequency resonance and having a little more range than normal, is it a good idea if i would insert a variable capacitor (around 34 pF) between my loading coil and my vertical antenna for my AM25? I've never experienced this but i'm planning to experience this in the next few months.
Any comments?
Yves Roy
Yves and others,
I have found by measurement the resistance load for the AM25 which maximizes the power in the load, but there is an issue about the tx. power adjustment which I need to fix and repeat my measurement. I should have a test report in a few days and will post it on this board.
Neil
Hello Neil,
I'm waiting for your report.
Thanks in advance. Yves