Posted on 01/23/2023 7:42:41 PM PST by Paul R.
Can thin diameter 75 ohm coaxial cable be connected to standard or large diameter low loss coaxial cable without creating odd impedance / transmission line reflections and / or losses that are more problematic than the reduced losses in the "better" cable?
SO...! While it might not make much difference, I am wondering if I could:
Scenario 1 - splice new copper braid / shield RG 59 or better 75 ohm coax cable (I have a lot of it) to the output of the antenna to link it to the amplifier. This would shorten the "thin" cable coming out of the antenna to 2" or so. BUT, will this potentially cause reflections at the splice and / or how would one avoid such?**
Scenario 2a - Shorten the thin cable between the antenna and amp to ~2", either by a splice that chops out most of the existing cable, OR, if I get brave...
Scenario 2b - Crack open the amplifier and wire the now 2" long thin cable directly to the solder points inside the case. Either way, to make up the increased length then needed from the amp to the TV I can use a std. female-to-female TV aerial connector and whatever length is needed of RG 59 or better cable with male ends on it to reach the TV.
"2a" again leaves me with a bit of a worry about the splice. In either 2a or 2b I'd have to extend the power cable - no problem!
2a and 2b allow me a lot of flexibility in remote mounting (distance to) of the antenna, including going up into the attic above the TV.
** The whole subject of RF transmission line impedance and reflection is "really wild" to a relative novice like me. This article
https://www.emisoftware.com/blogs/cabling-affects-emi-unexpected-ways/#:~:text=As%20cable%20length%20increases%20from,the%20impedance%20is%20very%20high.
makes me wonder how it would not be a matter of luck, or bad luck, that one or more TV stations one was trying to receive would not fall into a null or (just as bad?) a peak?
*The (presently unamplified) outdoor antenna gives me grief sometimes on strong signals too. I may need to put a LTE filter on it, between the antenna and the amplifier (which doesn't have LTE filtering) I had up there / would try again up there. A springtime project...!
Oh, good grief - I forgot to change the title to “Monday”!
(Started this question last night.)
Yes!
Any tricks to minimize impedance problems / reflections?
No!
Yes!
Maybe?
Prolly need some of them cathode diode transistor resistor capacitor thingys...
...and a roll of electrical tape.
I always got my best gain with 300 Ohm flat wire cable, and resent the day it started going to 75 Ohm coax. More interference, but more signal, too.
I have never found an RF amplifier that did much for the signal. Back in the pre-cable days, my father employed a device from Jerrold known as a trap, which was used to improve weak signals close to strong ones. In our case, Channel 8 (WNHC-ABC, New Haven) was overpowering channel 9 (WOR-Ind, New York) and Channel 7 (WABC-ABC, New York). That sometimes worked depending on atmospheric conditions, but it didn’t really strengthen the signal, only made the existing signal more useable.
You can donit but at these frequencies you’re not going to see much improvement and depending on your splices could make it worse.
If we still had a Radio Shack, they would have a connector for that.
Well, that last is what I was thinking, and am hoping to find guidance on, essentially, “proper splicing”.
I’d have to splice to get the antenna up in the attic regardless, so, eliminating the thin cable might not be much of a bonus, but, IF splicing didn’t cause trouble, it seems like I might as well use lower loss cable...
Any guesses on the losses in .11” dia. 75 ohm coax @ DTV UHF band frequencies?
As long as both cables are 75 Ohm you will have a good match, but you also have to use correct connectors, usually F series for RG-59 coax or RG-6 (low loss, a little bigger coax). RG-6 IIRC take a little different connector to match the larger wire diameter, but has the same F-series end.
Coax impedance is determined by relationship of size of the center wire in comparison to the ground shield surrounding it. As long as the size ratio stays the same, you can have a smaller or larger coax that is still 75 Ohm. That’s why you need the proper connectors to keep that relationship at the junction so there is no change in impedance at the junction.
I guess 75 ohm became more prevalent with the increase in interference sources...
At my parents we had a close & strong FM station that wreaked havoc with other FM signals and trying to get a distant VHF Channel 6 station that was right in line with that FM station. I knew the engineer for that strong FM station and he made us a custom notch filter to knock down his own station. :-)
I wouldn’t cut the factory cable on your device.
Larger or smaller cable won’t make much difference until you get into long runs, just make sure you match the impudence to maximize power transfer.
I use a good quality RG-6 coax for long runs. Lowes sells a kit with F connectors, cable strippers, crimper and cutters in a plastic case for about 25 bucks.
75 ohm became more prevalent when cable TV became prevalent. Using 300 ohm twin-lead with cable TV would cause serious interference, both to the cable TV signal from external transmitters, and from the cable TV signal to external receivers.
Due to cable TV, houses started to be pre-wired with 75 ohm coax instead of twin-lead. I actually once lived in a house that was pre-wired with twin-lead, built in 1983...there was no cable TV in that area back then.
When TV manufacturers decided to offer cable-ready TVs that could be connected directly to the cable line with no converter needed—the input had to be a 75 ohm F connector.
For a while, probably from the mid 80s to the early 90s, TVs and VCRs were being made with 75 ohm F inputs for VHF/cable TV, and 300 ohm inputs for UHF. Then after that they did away with the 300 ohm input entirely and the 75 ohm F input was for VHF/cable TV/UHF.
Radio Shack (when there was a Radio Shack) used to sell a whole selection of signal splitters to deal with this. They had ones that took a 75 ohm input and split it out to 75 ohm VHF and 300 ohm UHF outputs.
They also sold signal combiners that took the 75 ohm VHF and 300 ohm UHF inputs and combined them on one 75 ohm output, so you could use an older VCR (with separate inputs) with a newer TV (with a single combined input).
Throw out the RG59, buy new cable - RG 6/11 - use quality connectors and just do your splice/run.
Should be good, assuming you know how to do a coax connector without noise...
Here are flat 75 ohm cable and an adapter.
https://www.amazon.com/300-ohm-twin-lead/s?k=300+ohm+twin+lead
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