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VHF-HI Circular Loop's Reflector not working?

3K views 5 replies 3 participants last post by  holl_ands 
#1 ·
Hi,
I build a Hi-VHF Circular Loop + Reflector ( Hi-VHF Circular Loop + Reflector ) but I am getting almost as strong signal from the back side as the front. I used 1/4 inch metal grading that I had from shifting sand. I know most people talk about 1"x2" fencing, but I thought I read that the 1/4 inch worked as well or better, but had more wind drag. (Which is not a problem as I live in a forest.) http://www.digitalhome.ca/forum/186...nt/192242-designing-antenna-twin-lake-mi.html Any thoughts as to what is happening.
Thank you for your time, and this great site.
Eric
 
#2 ·
Unless you use a spectrum analyzer or a signal meter that actually measures the power of the signal(s), you won't see much of a difference until you're right on the edge of reception. If you're using the simple signal meter as incorporated into most TV sets, you probably won't see much difference.

There's not much front/back or front/rear differential, maybe 10-15 dB or so, depending on which specific version you built and what channel you're on.
 
#3 ·
It seamed I was getting better signal reception from the back side than the front, but it could just have been the weather. I was barely picking up channels from across Lake Michigan, while the channels I was trying to get (Antenna pointed to and 3-4x closer) I was not. If there is not that much difference with the screen, then that is O.K. I has thought that the screen stopped most of the signal. Thanks.
 
#4 · (Edited)
IF I understand it correctly, you THINK that the Hi-VHF Loop+ScreenReflector has poor F/B Ratio performance because you pick up a mostly OVER-WATER (Ch8 at 256-deg???) station [which can benefit GREATLY from DUCTING, esp THIS time of year] better than those completely over LAND???? Is there any other indication of poor F/B Ratio Performance??? What happens when you rotate the Antenna 180-deg in the opposite direction??? I tend to agree with your guess that it's the "Weather"....

It's probably not an issue, but is it possible that the 1/4-in Screen Material is NOT made of conductive metal??? IF in doubt, perhaps you can check it with an VOM Meter.

I don't think you have enough Antenna GAIN to pick up Hi-VHF Ch7 (NBC, NM=8.9 dB) and Ch8 (CBS, NM=3.0)....but before upgrading the Hi-VHF Antenna, there are some other issues that need to be addressed.

If you are still using the AntennaCraft 10G212 Preamp with extremely HIGH 30dB Gain...THAT can be a BIG problem for receiving weak Hi-VHF Stations. Although FMFool is also off-line at the moment, I can see that there are numerous FM stations within a dozen miles of your zipcode....the second Harmonics of which fall on-top of various Hi-VHF Band channels....very strong Ch13 and strong Ch11 can also generate 3rd Order Intermod Distortion products on top of other Hi-VHF Channels.

1) First, you should insert a FM FULL-BAND FILTER between the Hi-VHF Antenna and the Preamp, either from Antennas Direct or MCM:
https://www.antennasdirect.com/store/FM_band_rejection_filter.html
FM Trap | FM-88 (FM88) | Distributed By MCM

2) You might also need to replace the A-C Preamp (which has probably ALSO been degraded by the weather) with a Very High Overload Preamp, such as the Antennas Direct Juice:
https://www.antennasdirect.com/juice.html
Or alternatively, the inexpensive RCA TVPRAMP1 or Channel Master CM-7778.

3) Hi-VHF Antenna Upgrade: It was unclear whether you were using a Single or Vertically Stacked Loops with Winegard SD 3700 RF Combiner...which is it???? If pointed to receive Ch7 & 8, Ch13 (and maybe Ch11) will probably come in anyway and Ch9 would be a duplicate to Ch8, so I'm guessing that you'll only need a High Gain Hi-VHF Antenna pointed in ONE Direction.

Note that I posted Vertically Stacked Hi-VHF Loop+ScreenReflector designs that eliminate the Loss in the RF Combiner, providing a bit more than 10 dBi Raw Gain (as much as the obsolete Y10-7-13 or YA1713):
V-Stack Hi-VHF Circular Loops

Also note that I posted the simple to build Hi-VHF Hourglass-Loop+ReflectorRods designs that provide nearly 10 dBi Raw Gain, which would be about 13 dBi Raw Gain in a Vertical or Horizontal Stack....interconnected by a Vertical (HVH) or Horizontal (HHH) Harness that I would have to run through nikiml's Optimizer if you chose to go that route. And they are relatively insensitive to Wire Size, unlike Hi-VHF Loops:
http://imageevent.com/holl_ands/loops/hivhfhourglassloop
 
#5 ·
This has been a long process, but I am just about done with my stacked High VHF loop antenna. I have the 1/4" screen, PVC frame, and 1/4" copper tubing circles all in place. I know I need to put the loop ends 1 1/2" apart, and connect both together with equal length cable. I have both the 300 Ohms Twin flat cable, 300 Ohms/75 Ohms Baums and RG-6 cable to connect them together. With Holl_ands comments on using the FM trap, my question how best to put it all together.
I was originally planning on connecting the two loops with 300 Ohms Twin flat cable to a 300 Ohms/75 Ohms Baum (but I was not sure if the length of the flat cable mattered other than being the same length) to the preamp, and down to the booster. But now I see I need to add the FM trap, which is fine. Yes, we do have a number of FM channels in the area, and channel 13 is very strong in our area. If Antennas Direct's Juice is the best preamp to get, then I will get that, as I don't want to keep going up and down my 40' TV Tower.
Finally for clarification. I have two antennas, my old High VHF loop antenna (which is what I was using when I was surprised in getting signals from the back of the antenna from across Lake Michigan) and the new stacked High VHF loop antenna. The new one will be replacing the old one, and pointed at about 149 degrees to try and get WOOD-TV - 7, WWMT - 8, WZZM-13, WXMI 19, WGVU-TV-11. Is there anything that I am missing?
Thank you again for all the great work you do.
 
#6 ·
Because of High Loss when Wet (Damp/Snow/Ice), I do NOT recommend using Unshielded 300-ohm Twin-Lead (and Shielded Twin-Lead would need the ENDS well-sealed to keep out moisture):
http://www.digitalhome.ca/forum/186...ganging-2-m4s-avoiding-balun.html#post2642954

In the post following mine, old sparks suggests making your own Equal-Length DIY 300-ohm OPEN-LADDER Transmission Lines, which do NOT have high Loss when wet.
 
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