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#1
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Incorporating a Terk TV-5 as part of a beam?
I live 50m from TV transmitters, want to enhance ch9 (187mHz) without
using a roof antenna and have a Terk TV-5 amplified set top antenna. Is it possible to place directors ahead of its dipole and a reflector behind to increase the forward gain and null some of the multipaths? Dick |
#2
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Richard Lane wrote:
I live 50m from TV transmitters, want to enhance ch9 (187mHz) without using a roof antenna and have a Terk TV-5 amplified set top antenna. Is it possible to place directors ahead of its dipole and a reflector behind to increase the forward gain and null some of the multipaths? Dick The TV-5 antenna is an angular figure 8 in the horizontal plane having an overall span of 64cms but measuring around the fig.8 periphery is 86cms. In order to give ~ 8dB gain I was proposing to place 2 directors in front of the fig.8 and a reflector behind with 0.15 wavelength spacings and was seeking comments on the idea. Dick |
#3
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Hi Richard
My gut feel is that adding elements to a set top antenna is a pointless exercise. I think your biggest problems to obtaining a sharp low null pattern wil be internal (room/building) reflections and detuning by being close to other objects. (Element detuning will affect gain/nulls and as soon a you walk near it or place it within a few metres of any objects it will affect the pattern badly) The elements will also lower the antenna input impedence. This may or may not be a problem to the following amplifier. I am suprised that with transmitters that far away (I guess 50 miles not 50 metres) you are even considering using a set top antenna. I assume you have a good reason for this so there is no point embarking on suggestions of an external one. If I was limited to using an internal antenna I'd make a 3 element yagi specifically for 187MHz and tune it for maximum front to to back. Given the bandwidth of a TV signal though I doubt I'd reach a good figure. I would follow this by a preamp also tuned to 187MHz. Even then I doubt I'd get pleasing performance with it internally mounted. The only other suggestion I can make is to go ahead and try it and report back to the NG. Keep in kind that you have to do a good job of tuning the elements you add (spacing and length) to get good nulls. You may in fact want to try a single quad loop reflector and add a air space tuning capacitor in the loop. The idea is to resonate it to about 5% higher than the operating frequency so you'll need to make it slightly larger than that again to allow for the capacitor. I apologies for not being much help! Cheers Bob VK2YQA Richard Lane wrote: Richard Lane wrote: I live 50m from TV transmitters, want to enhance ch9 (187mHz) without using a roof antenna and have a Terk TV-5 amplified set top antenna. Is it possible to place directors ahead of its dipole and a reflector behind to increase the forward gain and null some of the multipaths? Dick The TV-5 antenna is an angular figure 8 in the horizontal plane having an overall span of 64cms but measuring around the fig.8 periphery is 86cms. In order to give ~ 8dB gain I was proposing to place 2 directors in front of the fig.8 and a reflector behind with 0.15 wavelength spacings and was seeking comments on the idea. Dick |
#4
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Bob Bob wrote:
Hi Richard My gut feel is that adding elements to a set top antenna is a pointless exercise. I think your biggest problems to obtaining a sharp low null pattern wil be internal (room/building) reflections and detuning by being close to other objects. (Element detuning will affect gain/nulls and as soon a you walk near it or place it within a few metres of any objects it will affect the pattern badly) The elements will also lower the antenna input impedence. This may or may not be a problem to the following amplifier. I am suprised that with transmitters that far away (I guess 50 miles not 50 metres) you are even considering using a set top antenna. I assume you have a good reason for this so there is no point embarking on suggestions of an external one. If I was limited to using an internal antenna I'd make a 3 element yagi specifically for 187MHz and tune it for maximum front to to back. Given the bandwidth of a TV signal though I doubt I'd reach a good figure. I would follow this by a preamp also tuned to 187MHz. Even then I doubt I'd get pleasing performance with it internally mounted. The only other suggestion I can make is to go ahead and try it and report back to the NG. Keep in kind that you have to do a good job of tuning the elements you add (spacing and length) to get good nulls. You may in fact want to try a single quad loop reflector and add a air space tuning capacitor in the loop. The idea is to resonate it to about 5% higher than the operating frequency so you'll need to make it slightly larger than that again to allow for the capacitor. I apologies for not being much help! Cheers Bob VK2YQA Richard Lane wrote: Richard Lane wrote: I live 50m from TV transmitters, want to enhance ch9 (187mHz) without using a roof antenna and have a Terk TV-5 amplified set top antenna. Is it possible to place directors ahead of its dipole and a reflector behind to increase the forward gain and null some of the multipaths? Dick The TV-5 antenna is an angular figure 8 in the horizontal plane having an overall span of 64cms but measuring around the fig.8 periphery is 86cms. In order to give ~ 8dB gain I was proposing to place 2 directors in front of the fig.8 and a reflector behind with 0.15 wavelength spacings and was seeking comments on the idea. Dick Today I made up a 2 director, 1 reflector "beam" out of 1/4" dowels wrapped with cooking foil, the reflector was planned to be 26cm to the center of Terk dipole, Terk to 1st director is 16cm as is 1st to 2nd director. The reflector is 80cm, 1st director is 73.6cm, 2nd director is 71cm. It appears to make a big improvement, however it "works best" positioned with the Terk about 2cm behind the 1st director! I have no field strength instruments but though "snowy" the sound is quite solid. I have not yet tried moving the whole assembly around the condo living room though given the Terk amplifier I should be able to use several meters of coax before the loss starts to show in noise figure. The "Association" does not allow external antennas and as I watch very little TV I resent paying $50/m for the one or two hours I watch each month. I am a retired microwave device engineer but not an antenna expert. Thank you for your interest in my experiment. Dick |
#5
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Hi Dick
Excellent! I use to chat to a fellow amateur many years ago that lived in a high rise apartment. He set up his 144MHz yagi on a short tripod and pointed it out the window. It was surprising how well it worked. I didnt mention but another effective antenna for unit and condo dwellers is aluminum foil antennas stuck to windows. This unfortunately may not work too well for TV because of the need to keep directivity Just for my interest what country/state/town etc are you in? Cheers Bob VK2YQA Today I made up a 2 director, 1 reflector "beam" out of 1/4" dowels wrapped with cooking foil, the reflector was planned to be 26cm to the center of Terk dipole, Terk to 1st director is 16cm as is 1st to 2nd director. The reflector is 80cm, 1st director is 73.6cm, 2nd director is 71cm. It appears to make a big improvement, however it "works best" positioned with the Terk about 2cm behind the 1st director! I have no field strength instruments but though "snowy" the sound is quite solid. I have not yet tried moving the whole assembly around the condo living room though given the Terk amplifier I should be able to use several meters of coax before the loss starts to show in noise figure. The "Association" does not allow external antennas and as I watch very little TV I resent paying $50/m for the one or two hours I watch each month. I am a retired microwave device engineer but not an antenna expert. Thank you for your interest in my experiment. Dick |
#6
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Bob Bob wrote:
Hi Richard The elements will also lower the antenna input impedence. This may or may not be a problem to the following amplifier. Cheers Bob VK2YQA Bob, I am in Port Townsend Wa and attempting to recieve PBS ch9 located in Seattle some 40 miles away, incidently ch13 (Fox) 38 miles comes in fine with the Terk alone but I am not enamoured of Murdoch's political bias even though he was a fellow Aussie. Dick |
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