2010-01-24, 05:30 AM
So I was experimenting some more with my 2xSBGH builds in my attic, and I concentrated so much on the digital reception, I never really paid much attention to the analog side of things. I was really surprised that I was picking up VHF-Lo channels and VHF-Hi to (see captures below). Now my Hauppauge signal monitor doesn't work on the analog side, so I don't know how powerful the signal is, and my antennas are not optimized for VHF reception. I'm just wondering how I'm getting any picture at all. Is it because I'm using a 36" wide 1"x2" reflector and that could be acting as a type of NAROD?
VHF-Lo Channel 5 (CBC analog)
VHF-Lo Channel 6 (Global analog)
VHF-Hi Channel 9 (CTV analog)
Could this be a good indication for channel 9 post transition?
2010-01-24, 11:51 AM
Marbles , There are quite a few threads i`ve read on this site that says alot of these antennas are UHF , not good for VHF and some are surprised that VHF can be picked up . Maybe in theory this holds but hey , everything i`ve had hooked up recieved VHF Lo & Hi with no problems at all . I use a 91XG and i get all the VHF`s off the back and sides with a watchable picture . I even get a Cleveland station 5-1 at 114 mi. almost all the time in the winter. I`ve built 2 gapless SBGH10`s and recieve all the VHF on them aswell (except Clev.). One is outside and one in an attic. I`ve also been playing with a GH8 without reflectors , (experimenting with feedpoint gaps & cable from balun position ) at 10' off the ground and i still get all the VHF aswell . The only time i couldnt recieve VHF was when i had a "UHF only" preamp on the XG . Even without a preamp on these antennas VHF still worked . Maybe the gain is quite low on VHF but still strong enough to recieve . So its no surprise to me that you recieve them .
2010-01-24, 12:53 PM
I think most antennas can get 'analog VHF' if it's a local or a very strong analog signal. You may be right about reflector being the VHF analog receivers, however the VHF digital I believe is not going to be as tolerant as analog was according to my results with VHF12.1 Erie. Without narods or a reflector on my antenna, I can not receive WICU 12.1 digital Erie, but I can recieve CHCH 11 analog Hamilton and CTV 9 analog Toronto,
2010-01-24, 02:21 PM
The GHs even without NARODs always have had some positive raw gain on vhf. But the very high SWR (the impedance mismatch) at those channels pushes the Net Gain well into the negative numbers.
From Ken Nists site :
Why show raw gain when net gain is what is important?
1. If atmospheric noise exceeds receiver noise then the raw gain is what counts. (This is rare for VHF or UHF but does occur in some neighborhoods.)
That may not be so rare when the tuner is near or inside a computer. Its fairly noisy in there, heh.
The impedance mismatch can also improve and degrade because of the condition or placement of the coax or balun, which is not modeled. Also the placement of the antenna and any nearby metal.
Ever since they quadrupled their original allotted output power, I can get channel 6 digital, 61 miles away, on my folding reflector-less SBGH well enough to lock about 80% of the time.
So it basically boils down to unique local conditions.
2010-01-24, 02:22 PM
We don't recommend the SBGH for VHF-LO because of the probability that it will not do what you are seeing. ;)
The point of the computer modeling R&D is to show in ideal terms how well/poorly a specific antenna can receive signals on certain frequencies. While the ideal results may show no appreciable gain or too high SWR on a certain channel there are umpteen oddities (usually very local to the receiving antenna) that might override all that.
As a real world example, my original CM4228 used to get CHEK on channel 6 booming in across the water from near Victoria, even though it was never designed for that and actually models quite poorly on VHF-LO.
So, enjoy it as you see it but don't be surprised if you'll be needing NARODs on it after CFTO's final transition to VHF-HI 9.