2009-02-02, 02:33 PM
With option C, you would have a lot of losses going to the second TV.
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2009-02-02, 02:33 PM
With option C, you would have a lot of losses going to the second TV.
2009-02-02, 02:38 PM
With only 12 ft of RG6? Or the fact that I'm hooking up two baluns to my antenna?
2009-02-02, 02:48 PM
A quality balun has about 1.5 dBi loss, the poorer ones have more. Plus the losses at splitter at the first TV.
2009-02-02, 02:52 PM
Sorry, I'm making this confusing...
Option C wouldn't require a splitter - I'm trying to get around this by using two baluns - instead of using a splitter (with at least 3.5 dB), I'm hoping to lose only 1.5 dB by using another balun. In effect, I would attach two baluns to my antenna so I don't have to use a splitter.
2009-02-02, 03:16 PM
Oh OK, I see now. With two baluns in parallel youll be changing the impedances so both sets will suffer, Im not sure how badly though.
2009-02-02, 03:32 PM
Two baluns in parallel == 1/2 the normal impedance, == 2X the usual SWR, == 1/2 the signal. But it will vary from channel to channel.
2009-02-02, 03:39 PM
Thanks guys, Option A it is!:D
2009-02-02, 04:54 PM
Also, don't forget that the majority of the signal loss (3dB) through a splitter is from half the signal going to each output. Ignoring the impedance mismatch, using two baluns you will still be splitting the signal between the two sets.
2009-02-02, 05:08 PM
Aye, Aye, Aye, meesa confused!
So, is "A" still the best option?
2009-02-02, 05:55 PM
Yes. Option A is still the best.
It isn't the best analogy, but think of it this way. If you have 1 liter of water and pour it equally into 2 coke bottles, it doesn't matter how you pour it (using a funnel or trying to aim at the hole), you can't have more than 1/2 a liter in each bottle (less if you spill some).
2009-02-02, 11:36 PM
The jury is out on just how much noise is saved by a 300-ohm input pre-amp (eg. Spartans), but it is probably better overall than a 75-ohm pre-amp + commerical balun. A home-made balun (very low loss) can change that equation.
I have I friend in Rosedale Toronto, with an SBGH in her 3rd floor loft window facing the CN tower. It has a Spartan 0064 (UHF/VHF combined, 300ohm input, 75ohm output) pre-amp attached, and there's no issue with any stations.
She loves watching PBS (17.x) as well as the locals.
I measured UHF loss through Baluns. Best was about 2 dB (+/- 0.5 dB) and
DOUBLE for most Baluns. Here are specs for a slightly better Balun:
Note the ambiguity re guaranteed Max (HA!!!!) vs "typical"....
First we have to understand that inside EVERY Preamp is a single-ended,
unbalanced low noise RF transistor....we won't find unbalanced, differential inputs....
So, to connect balanced output from a typical antenna, a BALUN is employed,
either external to the Preamp....or is internal....as seen inside C-M Spartan:
"OTA Reception Files" has specs & measurements re Antennas, Baluns,
Splitters, Preamps, et. al. including a copy of the aforementioned Preamp Chart
(modified www.solidsignal.com chart) and a derivation of the Maximum
Input Signal level consistent with no degradation to SFDR (Spurious Free Dynamic
Range). Follow links in "DTV Preamp Signal Overload Calculator - Rev K" for
additional supporting information and examples.
Consider manufacturer specs for EQUIVALENT 75-ohm vs 300-ohm Preamps:
W-G AP8275 (75-ohm) vs AP8283 (300-ohm): VHF & UHF NF are same
W-G AP8780 (75-ohm) vs AP8783 (300-ohm): UHF & VHF NF are 1.0 dB lower for 75-ohm
W-G AP8700 (75-ohm) vs AP8703 (300-ohm): UHF & VHF NF are 1.1 dB lower for 75-ohm
CM-0068 (75-ohm) vs CM-0265 (300-ohm): VHF & UHF NF are same
CM-7778 (75-ohm) vs CM-0064 (300-ohm): VHF & UHF NF are same
Only two on the above list indicate a small 1.0-1.1 dB improvement with 75-ohm input.
All the others are THE SAME NOISE FIGURE. Other than the two W-G exceptions,
I'm convinced none of these specs include the loss through an external Balun.....
and ALSO exclude the loss through the internal Balun. Otherwise the 300-ohm
input type would ALWAYS have a 1-2 dB higher NF (Noise Figure).
2009-02-02, 11:56 PM
Fair enough - thanks:)
Holl_ands - can you help shed some light on these max input numbers?
It's all explained in "DTV Preamp Signal Overload Calculator - Rev K":
Look in "OTA Reception Files". Follow links in the spread sheet to find additional
supporting explanations, discussions and examples.
Once you've absorbed that, I'll be happy to entertain questions....
2009-02-04, 10:12 AM
Ok guys, hopefully a quick question:
I installed a spartan preamp on Monday night. Perfect reception. Transformer is mounted to the wall as it wanted to be.
Get home Tuesday night... I only get three channels (less than wen I had no premp at all). I run a multimeter, the output going outside is reading 20.1VDC. So it looks like the power supply is working. I unplug the cable going outside to the antenna/preamp, reception dies completely (ok, this was expected). I unplug the power supply.... reception does not change at all.
I have not tried bypassing the power supply yet and running it straight out the wall (and subsequently up to an unpowered preamp) Which leads me to....
Sooooo. With a Channel Master Spartan 3 preamp... when it is unplugged, does it still pass a signal? ie: if the box out on the mast dies, does it pass a signal at all? What I'm trying to figure out before navigating the ladder in the snow again is if I have an unplugged cable possibly between the preamp and antenna (how the hell would that happen?!?!) or if I have a dead box.
I haven't tried re-aiming yet as I thought I had the sweet spot and no time to do that just yet.
Granted as well, last night the cloud ceiling was at 2.2KM, this morning at 3KM (no reception difference)
2009-02-04, 12:02 PM
I am not certain about the Spartan, but most pre-amps will block all signals when un-powered.
2009-02-04, 12:16 PM
Thanks roger 1818...
That's what I figured. I must be receiving signal from somewhere up the chain towards the preamp - just the really strong ones that can be picked up with a paperclip are making it down.
I have a barrel connector/wall plate on the wall that I can't bypass, but one more outside at the base of the mast (that is heat shrinked). I do all my own connections with compression ends, but something must be loose somewhere.
I found another post where the guy replaced them all and it started working in a similar situation... I'll just get up on the mast and see if the preamp is getting the 20VDC at the connection there and work my way down the pole if it isn't - if it is getting power, maybe my connections to the preamp are shoddy.
2009-02-04, 02:58 PM
Keep in mind that it needs both +20V (or whatever) on the centre conductor, as well as a good return (ground) connection back to the PSU on the coax shield.
2009-02-04, 03:02 PM
Get home Tuesday night... I only get three channels (less than wen I had no premp at all). I run a multimeter, the output going outside is reading 20.1VDC. So it looks like the power supply is working.
I had a problem here once, involving a Spartan pre-amp. Reception was noisy and not as good as it ought to have been.
But the problem turned out to not be the pre-amp, but rather one half of the 300ohm twinlead had broken off where it connected to the pre-amp.
Originally I had stripped the leads of the 300ohm for connecting it (which weakened it), but this is not necessary with a Spartan. They use little teeth that are supposed to be able to puncture/connect right through the twinlead.
But the Spartan boards are not too robust around the terminal screws, so be gentle. Before I reassembled it, I removed the board, flipped it over, screwed the screws all the way in, and then added lots of extra solder around them, to give more bite to the screw threads.
Works perfectly now, and is a lot stronger mechanically, too.
2009-02-04, 03:20 PM
I'll check those too. If I have to go up, I might as well do the whole thing.
I don't *think* I'm going to have a problem with the 300 ohm leads, mine came pre-stripped from CM. Having said that, I also took the step before installing it to run the leads out of the preamp, to the underside and ziptied the 300/75 ohm balun to the underbelly of the preamp - so there *shouldn't* be any stress on the screw-terminal contacts. But who knows.
Either way, I'll keep you all posted. It was a nice setup while it was working!
Speaking of which... I need to pickup more zipties. Note to anyone: Don't buy them at radio shack / the source. I have found that 2/3 of the last 4 batches of black zipties snap off at the head in near-0C temperatures with little to no pressure on them. Really cheap.
2009-02-04, 06:17 PM
Yeah.. not all zip-ties are made of nylon, it appears.
The black ones from the Borg (H.D.) seem okay, though.
2009-02-04, 08:04 PM
The ones from TheSource say they are made of nylon ;)
I doubt it... anyhow.
Thanks mlord and everyone, it was a bad connection. I got up the pole, no voltage. Whatever, secured everything a little better while I was there. Got down to the splice at the base of the pole, one was bad. Good enough for reception, bad for carrying voltage. I made a bad compression fitting, it had bulge! what did I do?!?! anyhow, it's fixed.
Oddly enough... reception is better now too heh. Whoulda thunk it.