From the hair brained idea department, what happens when you hook a 220' vertical long wire antenna to an exciter's composite input and see the modulation meter go to 50%? I tuned above and below the exciter's carrier frequency and "logged" WTLC 1310 5KW from 29 miles, WCSI 1010 330 watts from 12 miles and WLW 700 50KW from 90 miles. So if you ever see an AM signal riding along on both sides of an FM carrier (for example, 1200khz above and below the FM freq), grab some double shielded cable & use it between your stereo generator and exciter...that may well fix it. I had this happen 32 years ago...has anyone else ever seen AM go for a ride on FM? Airchecks of my 3 "loggings" are at http://bob-on-the-job.com/AM via Composite Input
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AM stations getting into FM composite input--confirmed.
- Thread starter BobOnTheJob
- Start date
Yup...over 20 years ago. As I recall, an extra ground strap between the processor and exciter fixed it for me. Exciter was in it's space in the TX and the proc just happened to be right next to it in the adjacent rack. Scraped some paint and secured the very short strap under the rack ears on each device. The TX and the rack were grounded with 2-in. strap at floor level, but it apparently wasn't enuff.
I'm not absolutely 100% certain that's what happened in this case, but probably...
It happened about 25 years ago in Wisconsin. Started hearing a top-40 station on 99.5 -- and another one -- actually, the same one -- on 96.5. After listening for a few minutes, I realized I was listening to the local 1480 AM station on the two FM frequencies. Remember, this was 25 years ago, no FM translators relaying AM stations yet!
I mentioned it to someone who knew the CE, and the 96.5/99.5 signals were gone the next day.
The 1480 station also owned a 98.1 FM, (different format) and the transmitters were in the same building. Do the math
(and note I was using a receiver with analog tuning - couldn't tell the difference between 99.5 and 99.58)
It happened about 25 years ago in Wisconsin. Started hearing a top-40 station on 99.5 -- and another one -- actually, the same one -- on 96.5. After listening for a few minutes, I realized I was listening to the local 1480 AM station on the two FM frequencies. Remember, this was 25 years ago, no FM translators relaying AM stations yet!
I mentioned it to someone who knew the CE, and the 96.5/99.5 signals were gone the next day.
The 1480 station also owned a 98.1 FM, (different format) and the transmitters were in the same building. Do the math
(and note I was using a receiver with analog tuning - couldn't tell the difference between 99.5 and 99.58)
A different twist, having an adjacent FM's STL to yours cause that FM to show up in the sidbands of an AM station.
The AM's STL was a double-hop composite model feeding an analog processor with a 10k to 500 ohm transformer.
That RCA Ampliphase fed a 4-tower array that was damn-near FLAT out to +-40 KHz. Bet it could pass a composite signal!
The FM's STL had drifted a bit.
The AM's STL was a double-hop composite model feeding an analog processor with a 10k to 500 ohm transformer.
That RCA Ampliphase fed a 4-tower array that was damn-near FLAT out to +-40 KHz. Bet it could pass a composite signal!
The FM's STL had drifted a bit.
I still use an analog tuned MacIntosh MR78...best radio ever made IMHO. Back to the topic...I'm surprised this doesn't happen more often. I had a question (off-board) posed to me...Why doesn't this happen more often? My first guess is that some exciters let the world into the composite input whereas better built ones may have a low pass filter. There's no reason to allow anything above 100khz into an exciter's input. But this Bext I have allows well above 1000khz in. And from past experience, so does the Gates TE-1...but hopefully all of those have been put out of their misery.w9wi said:
Had the same problem in Jacksonville, Springfield, Vermont with Gates TE-1 exciters and in Boston with an MX-15 all with FM stations on a co-owned
AM tower. Solutions varied from meticulous grounding techniques to change in length of composite input audio cable from stereo generator to double shielded
composite cable grounded at one end (just like audio cables). Also remember the problem with a BE FX-50
AM tower. Solutions varied from meticulous grounding techniques to change in length of composite input audio cable from stereo generator to double shielded
composite cable grounded at one end (just like audio cables). Also remember the problem with a BE FX-50
There's an AM with an FM translator near here that does NOT have the problem. They reportedly are using a Crown transmitter. Wonder if Crown's are immune from the problem or if they just got lucky?chrish said:
In the Wisconsin case, what I heard the solution was was to simply close the exciter drawer -- apparently some work had been done that required pulling the exciter out of the rack, and it didn't get put back in.
Given how rarely this happens, I think probably keeping all your shields in place is usually quite adequate to prevent this issue.
Given how rarely this happens, I think probably keeping all your shields in place is usually quite adequate to prevent this issue.
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