Improving the F/B ratio of a UHF TV 8 bay bowtie antenna - Page 1
Forum section: electronics / rf-microwave

author: videobruce
author_rank: Frequent Contributor
timestamp: November 12, 2020, 11:21:58 pm
content: This isn't Amateur Radio, but television reception.The antenna(s) discussed are from Channel Master. Similar model number, similar basic design, but slightly difference performance. They are the 4228A (1st) & the 4228HD (2nd), both are 8 bay- bowtie reflector antennas. My preferred antenna for UHF TV reception.Long story short, between the original (US made) 4228A with web screening as a reflector, this newer cheaper version has 2 sets of 8 horizontal tubes but doing so, reduces the F/B ratio noticeable. Compared to the original when I swapped antennas, I'm getting phase cancellation of some stations that are 180 degrees apart from a smaller antenna aimed at those which I didn't have trouble before.Before anyone asks, the swap improved reception of certain stations facing NW at the cost of the ones SE due to cancellation from a poorer F/B ratio.A classic "robbing Peter to pay Paul" scenarioThe question is; anyone having suggestions to improve the F/B ratio?? Being aluminum, soldering to that won't work and I question just wrapping/tying some type of screening (similar to what CM used on the original design) to the existing reflector.FWIW;I did some close measurements between the original (1st) and the replacement (2nd) 4228 and found;VHF reception (at least on channels 7-9) was less,UHF channels below 29 were 1-2 db greater,Channels 31-36 were 1-2 db less.The comparisons were taken less than 3 hours apart to minimize reception differences.I attached 4 bay versions of these two designs. The one on the left is the current, the one on the right is the original (ignore the one in the middle). The 8-bay version are 2 4 bays side by side.

author: cdev
author_rank: Super Contributor
timestamp: March 04, 2021, 06:35:14 pm
content: horizontal wires at slightly longer than the length for a dipole at the lowest operative frequency should be adequate. Chicken wire or (indoors only) aluminum foil taped or glued or stapled to cardboard would work.

author: videobruce
author_rank: Frequent Contributor
timestamp: March 04, 2021, 06:54:22 pm
content: I added medium size opening screening to the reflector. Electrically securing it at 4 points with bolts, nuts & washers for an electrical connection to the mast (thou I don't believe that was really necessary). The rest was with cable ties. It did nothing to improve the F/B ratio after taking readings before and after forward, then the back side of the same channels.I wasn't expecting that at all.

author: ocw
author_rank: Regular Contributor
timestamp: March 04, 2021, 07:57:04 pm
content: It can be hard to see depth in pictures taken like that. The screen/element spacing should be about about 4 inches (10 cm) for UHF reception and more than twice that for VHF. That antenna is not made for VHF without everything being larger. As is it will just work as a poor VHF antenna.

author: videobruce
author_rank: Frequent Contributor
timestamp: March 04, 2021, 10:36:53 pm
content: Those 'bow tie' antennas vary from 3" to 4 1/2" in depth. 4 - 4 1/2" seems to be more the norm from my measurements.The the lack of separation for VHF would decrease the F/B ratio, correct?

author: ocw
author_rank: Regular Contributor
timestamp: March 04, 2021, 11:19:18 pm
content: Lack of separation would decrease the F/B, gain and impedance of the antenna. As per most other antenna specifications, optimum spacing depends on frequency/wavelength.The picture makes your spacing look significantly less than 4". 4" is the spacing for Channel Master 4221 and other UHF bowtie antennas.

author: videobruce
author_rank: Frequent Contributor
timestamp: March 05, 2021, 03:37:47 pm
content: I did measure and yes it's a 2d pic.The one in the middle I believe was the oddball, it's a newer version of the one on the left which MCM electronics use to sell before they were bought out. The one on the right is the 'real McCoy' CM 4221.

author: cdev
author_rank: Super Contributor
timestamp: March 05, 2021, 04:30:03 pm
content: That actually seems to me that it would be inexplicable or even impossible if there was really a conductive grid (like chicken wire which lots of people use as an RF reflector with no issues)Are you sure? (that the F/B ratio did not improve)Thats a head scratcher. What about the distance between the bowtie elements and the reflector? (or between the driven elements and the reflector) Maybe that is important in this setting. Do you have a VNA?Maybe there is more optimization you could do on reflector distance?

author: videobruce
author_rank: Frequent Contributor
timestamp: March 05, 2021, 04:51:29 pm
content: It was wire screening I got from a 'Home Center' a few years ago for some basement windows to put under regular window screening for strength. Galvanized steel I believe.I specifically took F/B ratios (channel readings facing the stations, then 180 degrees from the backside to those stations) before and after on various channels, the difference was around 20db for both which I understood was the advertised F/B ratio of the antenna. I wholeheartedly agree that the result, or lack of, I didn't expect.A pic is worth 1k words (note the SS bolt/washer/nuts at the bottom)

author: ocw
author_rank: Regular Contributor
timestamp: March 05, 2021, 06:06:04 pm
content: What is the reason for improving the F/B? Do you have some interference from same channel stations more than 120 degrees off your desired stations?If you came up with a 100 dB F/B antenna you would have to deal with multi-path reception of those back stations which might be 20 dB below its direct path signal. So, it might not improve things over one with 20 dB F/B.Your vertically stacked bowties achieve gain be reducing the elevation reception lobe width. Horizontal spacing will narrow your receive azimuth width while having a better chance of improving your F/B ratio. However, wideband antennas can have compromised performance on individual channels.

author: videobruce
author_rank: Frequent Contributor
timestamp: March 05, 2021, 07:12:09 pm
content: Local stations 180 degrees are causing cancellation between 2 antennas facing opposite directions. Reception off ant A, begins to cancel desired reception off ant B.Reducing gain is defeating the purpose of the main antenna.This situation has only got worse in the past 30+ years I've been dealing with it thanks to the 'toy phone' industry severely reducing the UHF TV band cramping more stations into a smaller spectrum.

author: ocw
author_rank: Regular Contributor
timestamp: March 05, 2021, 07:27:52 pm
content: It sounds like you are combining two antennas on one feedline. If both antennas receive a significant signal from the same station, the antennas need to be spaced properly with the proper coax length to each antenna. Or the antennas need to have a narrow azimuth so that only stations in one direction are received. If preamp overload occurs due to a strong local station either a better quality preamp having a wider dynamic range is needed or a channel notch filter can be added.

author: videobruce
author_rank: Frequent Contributor
timestamp: March 05, 2021, 07:34:16 pm
content: Yes combining 2 antennas 180 degrees apart for stations 180 degrees apart.But no preamp, it would never work.I have custom tuned traps for three overpowered stations that allows a Winegard DA1018 18db DA to handle it.A preamp would be toast, literally.

author: cdev
author_rank: Super Contributor
timestamp: March 05, 2021, 08:07:18 pm
content: A tunable filter or coaxial stub notch filter sounds like the way to go.

author: ocw
author_rank: Regular Contributor
timestamp: March 05, 2021, 08:28:06 pm
content: Your distribution amplifier has an 18 dB preamp in it. Since it is an older unit it has frequency response up to 1000 MHz. It might get overloaded by a local cell phone site or anything else close operating from 40 - 1000 MHz. I have a 20 dB antenna mounted quality preamp, two close TV transmitter towers but have no problem receiving distant TV stations.It just takes a 15 dB SNR or signal-to-interference ratio to have DTV reception, so 20 dB F/B should be adequate unless both stations are on the same channel with one station much stronger than the other. I have channels where I have the choice of two or three stations depending on the heading of my directional antenna (log periodic).

author: videobruce
author_rank: Frequent Contributor
timestamp: March 05, 2021, 10:18:48 pm
content: I think you missed the problem. It's not duplicate stations, it's both antennas picking up the same station.How do you figure a DA has a "preamp" in it?

author: videobruce
author_rank: Frequent Contributor
timestamp: March 05, 2021, 10:20:18 pm
content: cdev;It's theONLYway to go. Again, 30 or so years and a dozen different filters worth of experience with them.

author: ocw
author_rank: Regular Contributor
timestamp: March 05, 2021, 10:38:56 pm
content: Look at the DA1018 spec sheet:https://www.wtfda.org/pdf/Winegard%20Tech%20Info.pdfIt has an 18 dB preamp before going to the splitter with its loss to the outputs. Since it was made for CATV it is not expecting a large amplitude difference between signals [making overload more likely].If installed correctly, reception by multiple antennas is the same as adding another bay(s) to your bowtie antenna. That will not cause a problem unless you combine them incorrectly. That can be corrected by changing the line length to one antenna or add a transformer for 180 degree phase change for one antenna.

author: videobruce
author_rank: Frequent Contributor
timestamp: March 05, 2021, 10:52:47 pm
content: There is nothing about a "preamp". It's just a spec sheet, no diagram.Considering there are separate pages for "preamps" (that mount outdoors next or near the antenna) and for "distribution amps" that are indoor only a distance from the antenna(s), I don't get how you called this a "preamp".Oh, thanks for the spec sheet, I have one of their cheaper 11xx series "DFA's" also I didn't have specs on either (due to age).

author: ocw
author_rank: Regular Contributor
timestamp: March 05, 2021, 11:05:56 pm
content: That link says, DA1018 * Gain = 18 dB. How do you think that the gain is created other than via a preamp? If there is no amplifier then a power cord is not needed.

author: videobruce
author_rank: Frequent Contributor
timestamp: March 05, 2021, 11:18:24 pm
content: An amplifier.The terms "DA" and "Preamp" refer to the device itself, not what's inside. This is how terms get mixed up.Man, its a DA. Just like in audio. Preamps and (Power) Amplifiers. Two different things.

author: ocw
author_rank: Regular Contributor
timestamp: March 05, 2021, 11:46:14 pm
content: If you want to just call it an amplifier (to a splitter making it a DA) then it will be easier to understand how it was not meant for the combination of weak and strong signals coming from an antenna and so is more prone to overload than a "preamp" would be.