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eHam Forums => RFI / EMI => Topic started by: KC2ZPK on December 11, 2014, 11:10:42 AM



Title: Strong FM broadcast station nearbye, will I have issues?
Post by: KC2ZPK on December 11, 2014, 11:10:42 AM
OK,

We are moving forward on a house and I am trying to get as much homework done before hand as possible. The house is about 3-4 miles from a 30KW FM broadcast station to the West. What issues will I have on HF?? Picking a different house is not really an option, will just have to deal with what I have for now.

thanks in advance
KC2ZPK


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: SWMAN on December 11, 2014, 01:12:34 PM
I live about 5 miles from a 50 KW station both AM and FM and I never notice anything strange or any interference at all. I use only a Kenwood TS- 570 and a vertical antenna. I also use a dipole at times.  73 Jim W5JJG


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: K4ISR on December 11, 2014, 01:33:10 PM
As long as the frequency itself, or its upper harmonics are outside of the ham frequency ranges, should be no problem. Regular FM radio even at the multiple order harmonics comes in around 1.6MHz, 3.2MHz, etc, all outside ham bands.


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: KH6AQ on December 11, 2014, 02:01:34 PM
There should be no problems on HF or VHF. Calculating for 30 kW, 3 miles, 6 di antenna gain, and free space the E-field at your QTH will be 400 mV/m and this should be no trouble at all.


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: AA4PB on December 11, 2014, 02:30:06 PM
As long as the frequency itself, or its upper harmonics are outside of the ham frequency ranges, should be no problem. Regular FM radio even at the multiple order harmonics comes in around 1.6MHz, 3.2MHz, etc, all outside ham bands.
Given that the FM band is 88 to 108 Mhz wouldn't the first harmonics all be 176Mhz or higher?


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: K4ISR on December 12, 2014, 07:29:38 AM
As long as the frequency itself, or its upper harmonics are outside of the ham frequency ranges, should be no problem. Regular FM radio even at the multiple order harmonics comes in around 1.6MHz, 3.2MHz, etc, all outside ham bands.
Given that the FM band is 88 to 108 Mhz wouldn't the first harmonics all be 176Mhz or higher?


Yes that was a typo (should have been GHz, not MHz), but the first level harmonics is still outside normal ham bands, coming in below 1.25m and above 2m.


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: KL7CW on December 12, 2014, 10:05:16 AM
In the rather unlikely event that the FM signal degrades your receiver, just install a regular low pass filter with a cut off frequency above 30 MHZ for HF or above 54 MHZ if you also operate on 6 meters.  IF you use a transceiver, make sure the LPF can handle the transceiver power.  If you use an amp just put the filter between the transceiver and the amp.  If your transceiver has a built in antenna tuner and you use non resonant antennas, then the filter would need to be placed in the receive path only.  Well engineered rigs should easily handle the FM, but not all rigs are well engineered.  There are other solutions, such as traps, you could explore if you do in fact have the need. Enjoy your new house !!
   Rick  KL7CW


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: KF7CG on December 12, 2014, 10:28:19 AM
If you are one that wants to keep a low harmonic output signal, you might already run with a low pass filter in line. I have one between my antenna tuner and my amplifier and rig. I always believe in putting appropriate filtering before my transmit antennas. Gets in the way of some SWLing but that is just the way it is.

KF7CG


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: N2CJ on December 12, 2014, 09:09:31 PM
I once operated from inside the WNEW-AM (50,000 watts) transmitter site (Carlstadt NJ) using my Kenwood TS-940S into a Cushcraft A4 on the roof. The AM towers were all within a few hundred feet. No problems at all.


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: AA4HA on December 14, 2014, 04:24:25 PM
I once operated from inside the WNEW-AM (50,000 watts) transmitter site (Carlstadt NJ) using my Kenwood TS-940S into a Cushcraft A4 on the roof. The AM towers were all within a few hundred feet. No problems at all.

That is one installation where you would not want to do the "tongue test" on the end of the coax (like you do on a nine volt battery to find out if it has a charge).


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: NO2A on December 16, 2014, 12:16:50 AM
I doubt you would have any problems from fm broadcast. I live not far from a 50kw fm station and a megawatt uhf tv tower,which also has a few ham repeaters on it. It doesn`t bother my station,but if you`re driving past it,it may cause the blocking effect on some fm radios anywhere near 101.5.


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: K5LXP on December 16, 2014, 07:55:54 AM
Part of whether there will be any "problems" or not depends on the radio being used too.  For a while I tried using my 706 on 6 and 2M and between the channel 2 TV transmitter 5 miles away and all the various and sundry commercial VHF transmitters on the air it was all but unusable on an outdoor gain antenna.  I made a couple of coaxial stub filters that helped out a lot but when I changed the 706 out to a 746Pro, all those IMD issues disappeared.

Mark K5LXP
Albuquerque, NM


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: WA2ISE on December 29, 2014, 12:56:38 PM
I once operated from inside the WNEW-AM (50,000 watts) transmitter site (Carlstadt NJ)

Heard long ago an urban legend that someone "borrowed" the 50KW transmitter and antennas of a station down for maintenance late one night to get on an HF band to chase some rare DX.   ;D
Must have been on 80, or maybe 160?  You probably wouldn't get 50KW, but you'd still be way over legal limit.  Seems the SWR would be horrible and you'd risk the finals, and or you'd have to jumper out any harmonic filters intended to keep the station off 160 or 80...   But if you did kill the finals, you'd just tell station management that they were due for replacement...


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: WW7KE on December 29, 2014, 02:18:39 PM
If your receiver has good RF selectivity and dynamic range, you shouldn't have a problem.  But if you use a cheap receiver, FM stations could still overload and/or cause intermod in the front end even though the HF bands are far below FM frequencies.

My Grundig Satellit 750 is one of those that crumbled when I lived 5 miles from a dozen 100 kW FM stations on top of South Mountain in Phoenix AZ, using the built-in whip.  An FM trap and external antenna cured most of the problem (an FM trap fixed a lot of digital TV issues as well).  I still have a few issues with the whip now that I'm about 25 miles away, but an external antenna with a tuner cures them.


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: KB2WIG on December 30, 2014, 09:56:49 AM


" urban legend that someone "borrowed" the 50KW transmitter and antennas "

I don't know about that legend. I do know of one BE who loaded up a AM Broadcast ant with something like a Johnson Ranger and got on 80m after signoff.....


klc


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: AA4HA on December 30, 2014, 11:06:36 AM
I once operated from inside the WNEW-AM (50,000 watts) transmitter site (Carlstadt NJ)
Heard long ago an urban legend that someone "borrowed" the 50KW transmitter and antennas of a station down for maintenance late one night to get on an HF band to chase some rare DX.   ;D
Must have been on 80, or maybe 160?  You probably wouldn't get 50KW, but you'd still be way over legal limit.  Seems the SWR would be horrible and you'd risk the finals, and or you'd have to jumper out any harmonic filters intended to keep the station off 160 or 80...   But if you did kill the finals, you'd just tell station management that they were due for replacement...
Sounds like a stretch. The exciter for the PA would be a few KW all by itself. Maybe you could jumper in an amateur rig to work as an exciter and jumper past the protective system and filters.

Even without all of that, it would be neat to attach in to their antenna system. That would be fun for a few nights.


Title: RE: Strong FM broadcast station nearbye, will I have issues?
Post by: KL7HQK on January 08, 2015, 08:40:42 PM
I had a 50kw FM station put up a 180' tower about 1/4 mile from my house two years ago. I've had no problems with it on two meters or HF. I don't use any other frequencies. It does get on top of some of the other more distant FM b'cast stations.