[SATLUG] OT: FM Blocker for HDTV OTA?

Geoff geofff at w5omr.shacknet.nu
Wed Jul 15 10:01:40 CDT 2009

Cheryl Holmes wrote:
> Do any of you know where my sister can get an FM blocker or how she make one
> so she can use her HDTV's OTA?  

A Favorite quote of mine from one of the Star Trek movies is when Cap'n
Kirk asked his ship's Chief Engineer, Scotty; "What did you do to that
ship?" Whereupon Scotty replied: "The more complicated you make the
plumbing, the easier it is to stop it up!"

That said, lemme give you a 'fer'instance'...

For grinz and giggles, in the apartment here in Houston, I have setup a
220MHz ham radio transceiver. The antenna is indoors as well.
I also happen to have a hand-held transceiver (Handi-Talkie) that covers
(among other bands) the 220MHz ham band. Last night, I was fooling
around with the HT. On my Analog Television, with digital converter box
and rabbit ears, that poor signal goes *poof* with 100mW (milli, 1/10th
of 1 watt) from the HT. 100mW is a very small amount of RF energy, but
because it is so close to the DTV channel we were watching, the RF
signal I was generating over-rode the television signal. It didn't help
either that both of the antennas were in the same room.

Ham Radio operators have long know the effects of RF Interference.

I am glad you asked this question, Cheryl. I was under the impression
that the actual broadcast frequencies of DTV signals had moved from
their analog counter parts. Not so... not quite.

*Are DTV signals broadcast on special frequencies?*

No. DTV broadcasts use exactly the same channels as regular analog
television. While many DTV stations are now occupying UHF broadcast
channels, broadcasters are allowed to move back to their original VHF or
UHF TV channel once the transition to DTV is complete.

The only caveat is that TV channels 51 through 69 will be auctioned off
for other uses at the end of the transition to digital TV. Stations who
originally had analog channels in this band will have to move, no matter

One potential problem with re-using low VHF (2-6) TV channels for DTV is
the possibility of interference from other signals during certain times
of the year. "Skip" may bring in distant broadcasts on the same channel
and create interference. Impulse noise is also a problem on low VHF
channels. What’s more, the physical size of low VHF and high VHF
antennas is much larger than that of a UHF antenna.

Tests so far seem to indicate that high VHF channels (7-13) are quite
well suited for DTV broadcasts, and many broadcasters plan to move back
to their high VHF channels at the end of the transition. VHF
transmitters also cost much less money to operate than UHF transmitters.
(source: http://www.avsforum.com/hdtvfaq/HDTV-FAQ.htm)

So... that had me looking for the frequency list of broadcast channels.

According to this list, Channels 7 through 13 are in the VHF Hi-band
range, from 174MHz thru 216MHz (we're getting close to 223.5MHz, aren't
we?). Because of my Ham Radio experience, I can tell you that there are
some rather trashy pagers that operate in the 150~160MHz range. Noaa
Weather Radio has a transmitter in San Antonio, on the NW side, near
Bandera Rd on 162.55MHz. There are other services that use Radio
Frequencies in that range below 174MHz.

What you are looking for is not a 'blocker' (in effect, that's what it
does, yes) but rather a filter. Specifically, in your friends case we
need a high-pass filter. A High-pass filter can be designed to filter
out frequencies -below- the bottom end of the digital tv band. So, a
High-pass filter that takes everything and filters it out, below 170MHz
would -help- but it's not going to block anything that operates close to
the broadcast frequencies of your DTV system. Police, Fire and
Ambulance, taxi-cab drivers, all kinds of commercial communications
occur from 450MHz up to 470MHz. Then there's the Government and the
Military which pretty much operate wherever they damn well please.

That being said, according to the chart at
http://www.dbsinstall.com/Broadcast/vhf_uhf_freq_list.asp, there are
6MHz gaps between Channels from Ch14 thru Ch69 and who knows who or what
is operating 'between the channels'.
(although I did read somewhere where Channels 52 thru 59 were to be
re-allocated after the DTV conversion. This is the 700~750MHz band
(range) of Frequencies.)

> She doesn't want to spend the money for cable or satellite, which is the only way to get TV on these HDTV's currently.
> One of my friends who is the chief engineer at a local TV station told me about these problems and how that would be the only resolution for her problems.  They have lost thousands of viewers because the DTV signals are being blocked by FM radio and other signals...he says the only way to get TV on them is to use a blocker, although this station is actively tring to do something abt this with the FCC.

Could you be more specific with what station, where her location is, and
what FM Station(s) she's hearing?

You might ask yourself "why is he interested in all that, if he's in
Houston?" Well.. it's because I lived in San Antonio for 49 and a half
years. My dad was the chief broadcast engineer for KITE Radio (back when
having an FCC First Class Phone actually *meant* something), he also
worked at KONO, KBER, KUKA, KAPE and a few other stations, back in the
'60's. I grew up in an RF environment, have been a licensed ham radio
operator for 25 years now, and I *know* that town.

Gimme some specifics.


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