Balun loss and the CM7777

[Tim58hsv]

I've been thinking lately that maybe a preamp such as the CM7777 isn't the best choice for antennas with 300 ohm downleads. Instead maybe a preamp with a 300 ohm input and a 75 ohm output would be a better choice due to the signal loss of the balun. From what I've read a baluns signal loss can run anywhere from 1+db to over 6 db depending on the channel so if it's a fringe channel your trying to pick up, even a 1 db loss of signal before the preamp could kill it.

So what do you all think?

Here's a link that purports to show average balun signal losses.
FM TV Baluns

 

[Fringe Reception]

Tim,

Your concern is spot-on and considering older TV sets use a balun on both ends of the coax, double the signal losses. Modern sets are 'cable-ready' with a 75 ohm input - but what does that actually mean? There may be a balun lurking inside.

I built my own coaxial baluns for most of my home-brew Yagis to cut the losses to an absolute minimum: they are designed to the same frequencies as the single-channel antennas are, thus there is far less loss on the antenna end and a resulting narrowed bandwidth of the antenna system which tends to attenuate (potentially interfering) signals on other frequencies.

Part of the problem is the impedence of any given antenna changes over the frequency range it receives and this is not a balun issue, but it further contributes to inefficiency. Using 300 ohm to 75 ohm 'store-bought' baluns and coaxial cable in most TV antenna systems is a given because it is usually not practical or advisable to use open-wire 300 ohm twin-lead - which also acts as a receiving antenna.

So, my advice is to use an antenna that collects plenty of surplus signal, accept the losses and go watch TV!

Jim

 

[Cutthecable]

Anyone know where you can buy 300ohm twin lead nowadays?

 

[No Static At All]

It is my understanding that 300 ohm preamps have an internal balun, so it most likely a wash.

 

[Tim58hsv]

Anyone know where you can buy 300ohm twin lead nowadays?

Radio Shack has 100' rolls for $20.00.

 

[n2rj]

Anyone know where you can buy 300ohm twin lead nowadays?

I bought some from radioshack. It was rather poor quality. I did buy some from ebay which was better.

 

[Tim58hsv]

It is my understanding that 300 ohm preamps have an internal balun, so it most likely a wash.

I read that somewhere recently too so I guess the best bet would be either a high quality preamp such as a Channel Master with a 300 ohm input or a CM balun and a CM preamp with a 75 ohm input. The reason I specify the CM brand is because they're suppose to have the best baluns and preamps with the lowest thru loss. Hopefully their internal preamp baluns are just as good.

And I believe their baluns are very good because...

Just today I pulled down the G1483 antenna and replaced the over priced Radio Shack balun with an old Channel Master balun. The result was that the signal on my weakest (and favorite) station, WBQC, went from just over half way up on the converter box signal meter to almost 2/3rds of the way up. There's that and the fact that a tv here that never picked up WBQC without the converter box picks it up now.

I guess the bottom line is that baluns are a necessary evil but the damage they do can be negated to some extent.

 

[SWHouston]

Tim, It kinda depends on the circumstances....

The CM7777 has an output of 23dbVhf/26dbUhf with Coax Power Inserter.

The 300 ohm (input) Preamps are...
Winegard AP4800 which is UHF only @ 28db and 110v separate power cable.
Channel Master CM0064 16dbVhf/23dbUhf with Coax Power Inserter, and
Channel Master CM0264DSB 16dbVhf/23dbUhf with Power Inserter.
(these are the only 300ohm that I'm aware of)

Given the drop in output, I'm not sure that you'd be gaining much
unless you were replacing a real crappy Balun.
Then, one should consider justification in the additional cost of Twin Cable and Amp.

I know that 300 is as low loss as you can get, but, I've never liked that stuff,
and given its vulnerability to interference, I think I'd stay with Coax.

PS:
I know there are some others of these amps, but keeping up with them (available/discontinued) is like killing Roch's. So, if you've found
one/others that are available, post um !

 

[Jim Navotney]

Virtually all tv antennas made today are still 300 ohm antennas.
This means that they all must use lossy baluns to use a 75 ohm feedline.
The best solution is clearly a preamp with a 300 ohm input and 75 ohm output but sadly no enterprising young businessman has taken advantage of this fact and ignored millions in sales.