LNA for my TV antenna

[Mac]

I live near several strong HDTV broadcasters, but for some reason, in one
room in my house (the one with the TV, unfortunately) I get very poor
reception. This is true for regular TV, too. The transmitters for regular
TV are in the same place as the HDTV ones.

I made a simple folded dipole antenna, and with it I get two HDTV
stations. With store bought antennas, I get nothing, so I guess the folded
dipole is reasonably effective. For the moment, all the stations are in
the UHF, so having a relatively narrow band antenna is OK, I think.

It seems like the easiest next solution is to try a low noise amplifier
between the antenna and the TV. So my question is this: Is there a simple
and possibly worthwhile way to make a LNA which can span the television
broadcast range, from 50-800 MHz. Should I just by a cheapo amp from Radio
Shack and be done with it? I have pretty much ruled out normal op-amps,
because the fast ones aren't all that low noise, and they don't cover all
the way up to 800 MHz. (If I'm wrong, let me know, by the way.)

But it seems that a single transistor set up as a common-emitter amp might
do the job if it is biased right. How hard would such a circuit be to get
right without using signal generators and network or spectrum analyzers? I
have access to those at work, but I'm pretty busy, and don't really have
time to fool around with extracurricular activities.

If this is workable, then maybe someone could recommend a specific
transistor, preferably one stocked by digikey, since I will be ordering
some other stuff from them soon anyway.

best regards,
Mac
--

 

[Robert Baer]

I live near several strong HDTV broadcasters, but for some reason, in one
room in my house (the one with the TV, unfortunately) I get very poor
reception. This is true for regular TV, too. The transmitters for regular
TV are in the same place as the HDTV ones.

I made a simple folded dipole antenna, and with it I get two HDTV
stations. With store bought antennas, I get nothing, so I guess the folded
dipole is reasonably effective. For the moment, all the stations are in
the UHF, so having a relatively narrow band antenna is OK, I think.

It seems like the easiest next solution is to try a low noise amplifier
between the antenna and the TV. So my question is this: Is there a simple
and possibly worthwhile way to make a LNA which can span the television
broadcast range, from 50-800 MHz. Should I just by a cheapo amp from Radio
Shack and be done with it? I have pretty much ruled out normal op-amps,
because the fast ones aren't all that low noise, and they don't cover all
the way up to 800 MHz. (If I'm wrong, let me know, by the way.)

But it seems that a single transistor set up as a common-emitter amp might
do the job if it is biased right. How hard would such a circuit be to get
right without using signal generators and network or spectrum analyzers? I
have access to those at work, but I'm pretty busy, and don't really have
time to fool around with extracurricular activities.

If this is workable, then maybe someone could recommend a specific
transistor, preferably one stocked by digikey, since I will be ordering
some other stuff from them soon anyway.

best regards,
Mac
--

Why not put an antenna in a decent place, use a 300 ohm to 75 ohm
transformer, a low loss version of RG59 to TV (which might accept the 75
ohm connector directly).
That is what i did; had about 70 feet of cable; worked like a champ,
even with weak stations over 100 miles away.

 

[Mac]

Why not put an antenna in a decent place, use a 300 ohm to 75 ohm
transformer, a low loss version of RG59 to TV (which might accept the 75
ohm connector directly).
That is what i did; had about 70 feet of cable; worked like a champ,
even with weak stations over 100 miles away.

That is on my list of backup plans, but I'm trying to avoid running cable
between rooms in my house, since I don't have a good way to hide the
cable, and I don't want to run it on the baseboards. It's nice to know I
can get away with the long cable run if I have to, though. ;-)

regards,
Mac

 

[John Woodgate]

<[email protected]>) about 'LNA for my TV antenna',

Should I just by a cheapo amp from Radio
Shack and be done with it?

Almost certainly, but maybe from Newark, for example rather than the
Shack. These things use hybrid amplifier devices specially for the job.
While you can buy the devices, the PC layout is critical and you won't
be able to make a unit for less that double the price of buying one.

 

[SioL]

I made a simple folded dipole antenna, and with it I get two HDTV
stations. With store bought antennas, I get nothing, so I guess the folded
dipole is reasonably effective. For the moment, all the stations are in
the UHF, so having a relatively narrow band antenna is OK, I think.

<snip>

You've got the money for HDTV, but are saving a few bucks on a good antenna?

Get a nice wideband log-periodic antenna and put it on the roof, as high as possible.
You won't need a LNA anymore.

You can get a nice wideband log-periodic to cover entire UHF band. But you may
need a few antennas extra for the lower bands. Perhaps you just need UHF?

You can't improve your S/N ratio when your signal is below ambient noise. No amplifier
can change that. A good antenna can.

Siol

 

[Fred Bloggs]

That is on my list of backup plans, but I'm trying to avoid running cable
between rooms in my house, since I don't have a good way to hide the
cable, and I don't want to run it on the baseboards. It's nice to know I
can get away with the long cable run if I have to, though. ;-)

regards,
Mac

The only appropriate use for an LNA in this application would be to
overcome cable feed loss from a remote antenna to the receiver-locate
the LNA at the antenna. Your reception is bad because the input signal
power is swamped by the internal noise on the receiver front
end-assuming the xmit distance is not so great that atmospheric noise
has swamped it-and you say this is not the case. Any LNA fed by the same
antenna will also similarly swamp the antenna signal with its noise- so
you end up piping larger S+N with S/N even worse, because you have added
LNA noise which should be comparable to TV front end noise, into your
receiver. You may get the signal power above the thresholds for locking
in a synchronous frame but the picture noise will still be bad.

 

[John Larkin]

I live near several strong HDTV broadcasters, but for some reason, in one
room in my house (the one with the TV, unfortunately) I get very poor
reception. This is true for regular TV, too. The transmitters for regular
TV are in the same place as the HDTV ones.

Maybe your problem is too much signal, not too little.

John

 

[Fred Bloggs]

Maybe your problem is too much signal, not too little.

John

Close but no cigar- most likely effect of too near would be that ground
wave overshoots his house before it has developed- no field strength to
be received. Too much signal would be characterized by reception of
satisfactory signal with no antenna at all.

 

[John Larkin]

John Larkin wrote:

Close but no cigar- most likely effect of too near would be that ground
wave overshoots his house before it has developed- no field strength to
be received. Too much signal would be characterized by reception of
satisfactory signal with no antenna at all.

Interesting concept: an antenna that transmits a powerful far-field em
wave with zero near-field intensity. You should post that idea to one
of the free-energy newsgroups.

I guess that explains why Paul's field strength meters don't work:
they're too close to the antenna to pick up any signal.

Really, you're so eager to contradict me that you do it without
thinking first. That's not effective.

John

 

[John Woodgate]

I read in sci.electronics.design that John Larkin <jjlarkin@highlandSNIP
techTHISnologyPLEASE.com> wrote (in <5a0j00l79itd3jtgab405ggoa8jojmpa2t@
4ax.com>) about 'LNA for my TV antenna', on Sat, 17 Jan 2004:

an antenna that transmits a powerful far-field em
wave with zero near-field intensity.

It's called 'umbrella effect', and is significant for antennas mounted
at high elevation on masts. The field strength at ground level close to
the mast is not very high because those sites are 85 or more degrees
off-axis of the antenna. The BBC TV antenna (1 MW) at Crystal Palace in
south-east London provides no more than a 'strong signal' (less than 50
mV/m IIRC) at the surrounding houses, not 100 V/m.

I hate to think what would happen to the buses at the terminus in
Crystal Palace Parade if the field strength were 100 V/m.

 

[John Larkin]

I read in sci.electronics.design that John Larkin <jjlarkin@highlandSNIP
techTHISnologyPLEASE.com> wrote (in <5a0j00l79itd3jtgab405ggoa8jojmpa2t@
4ax.com>) about 'LNA for my TV antenna', on Sat, 17 Jan 2004:


It's called 'umbrella effect', and is significant for antennas mounted
at high elevation on masts. The field strength at ground level close to
the mast is not very high because those sites are 85 or more degrees
off-axis of the antenna. The BBC TV antenna (1 MW) at Crystal Palace in
south-east London provides no more than a 'strong signal' (less than 50
mV/m IIRC) at the surrounding houses, not 100 V/m.

But I doubt that 50 mV/m will require an extra LNA to get decent
reception.

I hate to think what would happen to the buses at the terminus in
Crystal Palace Parade if the field strength were 100 V/m.

My old Fiesta (known in Ann Arbor as the Ford Fiasco) had a cheap
after-market radio and ratty speaker wiring. When I drove over Twin
Peaks on my way to work, near the base of Sutro Tower - maybe 25 MW
total - my speakers would squwak and buzz, with the radio turned off.
The maximum seemed to be at roughly 45 degrees blast angle, but the
terrain is messy there so complicates things.

http://www.jimprice.com/sutro/

People who live near the base of the tower do have problems with the
huge field strength, and not just with their TVs. Lots of telephones
and other gadgets don't work up there.

John

 

[Tim Shoppa]

I made a simple folded dipole antenna, and with it I get two HDTV
stations. With store bought antennas, I get nothing, so I guess the folded
dipole is reasonably effective. For the moment, all the stations are in
the UHF, so having a relatively narrow band antenna is OK, I think.

It seems like the easiest next solution is to try a low noise amplifier
between the antenna and the TV.

I disagree. What you probably want is a Yagi antenna pointed at the
stations of interest. As you point out, UHF narrow-band antennas are
small and easy, so building a Yagi for each station you're interested in
is reasonable.

The next step is to use better feed cable from the roof to your TV.
RG-6 is cheap and really quite good.

If this is workable, then maybe someone could recommend a specific
transistor, preferably one stocked by digikey, since I will be ordering
some other stuff from them soon anyway.

If you really want to build a LNA, the NE34018 is cheap and has a very
very good noise figure. And it's stocked by Digikey.

NEC has some application notes with suggested
PC board layouts. It's generally used in the low GHz region, but will be
great for UHF too.

Tim.

 

[John Woodgate]

I read in sci.electronics.design that John Larkin <jjlarkin@highlandSNIP
techTHISnologyPLEASE.com> wrote (in <m75j00tmub0dk4l8m3j0joj2qqjv7qpb3n@
4ax.com>) about 'LNA for my TV antenna', on Sat, 17 Jan 2004:

My old Fiesta (known in Ann Arbor as the Ford Fiasco)

Was it not much bottle?

had a cheap
after-market radio and ratty speaker wiring. When I drove over Twin
Peaks on my way to work, near the base of Sutro Tower - maybe 25 MW
total - my speakers would squwak and buzz, with the radio turned off.
The maximum seemed to be at roughly 45 degrees blast angle, but the
terrain is messy there so complicates things.

http://www.jimprice.com/sutro/

People who live near the base of the tower do have problems with the
huge field strength, and not just with their TVs. Lots of telephones
and other gadgets don't work up there.

Ah, well, the BBC had to spend quite a lot of effort to meet the
government's requirements about protecting the surroundings from high
field strengths, not for any biological reason (it was long before the
current scares) but simply so that radio, TV and telephones continued to
work.

The original separate ITV mast had to have re-designed antennas shortly
after going to full power (IIRC), because the umbrella leaked.

 

[John Larkin]

The original separate ITV mast had to have re-designed antennas shortly
after going to full power (IIRC), because the umbrella leaked.

The prople who built Sutro Tower found it more efficient to just buy
off the politicians.

John

 

[Zak]

Interesting concept: an antenna that transmits a powerful far-field em
wave with zero near-field intensity. You should post that idea to one
of the free-energy newsgroups.

IWell, ISTO that close to an antenna magnetic and electrical fields can
be 'out of balance'. If you put a 'magnetic' and an 'electrical' antenna
(a small high-current loop, and a tiny dipole) close together, the
coupling will be inefficient. At larger wavelengths things will be
better, relatively. But I guess less distance will always mean a
stronger signal (minus radiation pattern effects...)


Thomas

 

[Fred Bloggs]

Interesting concept: an antenna that transmits a powerful far-field em
wave with zero near-field intensity. You should post that idea to one
of the free-energy newsgroups.

I guess that explains why Paul's field strength meters don't work:
they're too close to the antenna to pick up any signal.

Really, you're so eager to contradict me that you do it without
thinking first. That's not effective.

John

He's in the far field but on a low point of the elevation gain pattern,
the ground wave has not formed, or he is in the shadow of a large
obstruction like a hill.

 

[Mac]

<[email protected]>) about 'LNA for my TV antenna',


Almost certainly, but maybe from Newark, for example rather than the
Shack. These things use hybrid amplifier devices specially for the job.
While you can buy the devices, the PC layout is critical and you won't
be able to make a unit for less that double the price of buying one.

Thanks. I'll take a look at Newark. I am not totally ignorant of the
layout issues involved, but I very much appreciate your warning. There are
times when I don't mind experimenting, but for the moment I am just trying
to find a solution that doesn't involve putting the antenna in another
room or on the roof. ;-)

Mac

 

[Mac]

Maybe your problem is too much signal, not too little.

John

I don't think so. Without an antenna I get nothing, and when I tune in
non-HD stations they look snowy, the classic sign of weak signal. Also,
the HD receiver has a signal strength meter, which is at about 40 percent
(whatever that means) on the stations I receive, and at 0 for the stations
I don't receive.

Mac

 

[Mac]

He's in the far field but on a low point of the elevation gain pattern,
the ground wave has not formed, or he is in the shadow of a large
obstruction like a hill.

This is probably right. Depending on the exact location of the antenna, I
may not have a clear line of sight from my house.

Mac

 

[Mac]

But I doubt that 50 mV/m will require an extra LNA to get decent
reception.


My old Fiesta (known in Ann Arbor as the Ford Fiasco) had a cheap
after-market radio and ratty speaker wiring. When I drove over Twin
Peaks on my way to work, near the base of Sutro Tower - maybe 25 MW
total - my speakers would squwak and buzz, with the radio turned off.
The maximum seemed to be at roughly 45 degrees blast angle, but the
terrain is messy there so complicates things.

http://www.jimprice.com/sutro/

People who live near the base of the tower do have problems with the
huge field strength, and not just with their TVs. Lots of telephones
and other gadgets don't work up there.

John

I live 1.5 miles soutwest of Sutro Tower. But in a room literally 20 feet
away from the HD receiver, I get all regular (non-HD) stations just fine
with a folded dipole made from 300-Ohm cable. In the bad reception room,
the regular reception stations come in but are very weak. Some of them
don't really come in at all. So I know the situation isn't hopeless. I
just don't want to lay cable between the two rooms.

Mac