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post #211 of 226 Old 05-25-2017, 08:12 PM
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The GE 34792 Attic Antenna has a very effective reflector for UHF and a real UVSJ instead of shorting stubs as in the RCA ANT 751R.





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post #212 of 226 Old 05-27-2017, 02:38 AM
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Quote:
rabbit73:....a real UVSJ instead of shorting stubs as in the RCA ANT 751R.
Like most VHF/UHF Combos, ANT-751 has a pair of Rods [Transmission Line] running from UHF Folded Dipole Feedpoint back to the Hi-VHF Section....there are NO "Shorts"....nor anything that would "separate" the Frequency Response of Hi-VHF and UHF Sections....whatever that means.....the very short "Stubs" extending along Boom from Feedpoint only affect UHF Impedance matching (i.e. SWR).

Does NOT Compute....Please Explain....

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post #213 of 226 Old 05-27-2017, 05:08 PM
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Quote:
Originally Posted by holl_ands View Post
Does NOT Compute....Please Explain....
I will try.
Quote:
Like most VHF/UHF Combos, ANT-751 has a pair of Rods [Transmission Line] running from UHF Folded Dipole Feedpoint back to the Hi-VHF Section....
Correct
Quote:
there are NO "Shorts"....nor anything that would "separate" the Frequency Response of Hi-VHF and UHF Sections....
Yes, there are. The 1/4 wave stubs are connected to the transmission line at a point on that line to short out the UHF signals collected by the VHF section without harming the UHF signals collected by the UHF section.
Quote:
the very short "Stubs" extending along Boom from Feedpoint only affect UHF Impedance matching (i.e. SWR).
Not true. If they were connected to the feedpoint, they would harm the UHF signals collected by the UHF driven element. In the case of the ANT 751, they are connected to the top of the Winegard Tetrapole (3/4 wave folded dipole) where the VHF transmission line is connected. That is a high impedance point for the UHF signals because it is 3/4 of a wavelength away from the feedpoint which is at the bottom of the Tetrapole. According to transmission line theory, the outer end of a 1/4 wave stub is a high impedance point, as it is at the ends of a 1/2 wave dipole.

This makes the inner end of the stub a low impedance point, which shorts out any UHF signals from the VHF section at that point.

This is a detail of the photo you took at CES 2009:


 
This is the Tetrapole from the Winegard patent:


 
This is a stub diagram with my notes added. Salvati doesn't mention that the stubs must be a quarter wavelength (or multiple of) away from the UHF feedpoint when a Tetrapole isn't used:


 
The RCA ANT 7511 uses a long dipole instead of a Tetrapole, so the stubs must be 1/4 wavelength away from the feedpoint. The single pair of thin stubs aren't very effective. The redesign of the 751 gives up a lot of gain according to my crude indoor range measurements:


 
 
 
I have attached a pdf of Isolation Networks by M. J. Salvati from his book TV Antennas and Signal Distribution Systems.

OK?
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post #214 of 226 Old 05-27-2017, 07:50 PM
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Next I took a look at the Antennas Direct EU385CF-1S Diplexer (UVSJ) that passes power on the UHF side.





The back was sealed with epoxy; another sample was sealed with solder and epoxy.

C1 is a blocking capacitor that keeps the DC power for a UHF preamp out of the VHF antenna if you are using separate antennas for UHF and VHF.

L1-L4 are part of the VHF low pass section, L5-L7 for the UHF high pass section.

L8, L9, and C2 are a low pass filter that provides DC power for the UHF preamp.
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post #215 of 226 Old 05-27-2017, 08:25 PM
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This is an old MACOM UVSJ that passes power on the VHF side.






L1, C1, and L2 are the lowpass VHF filter; C2, L4, C3, and L3 are the UHF highpass filter.
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post #216 of 226 Old 05-29-2017, 03:19 PM
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Seems my 751 did a better job than my CM 2020 does. I may remove the 2020 and put the 751 back up.

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post #217 of 226 Old 05-30-2017, 03:50 PM
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Some of these UVSJ devices may be becoming scarce, especially with many Radioshack locations now closing down. Perhaps Channel Master should consider making a new version. It also seems more difficult to find a Holland or Pico-Macom version. But may be good to check out the newer Stellar Labs version from MCM. I believe it also passes power on the UHF side.
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post #218 of 226 Old 05-31-2017, 01:56 AM
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UVSJ's from Pico-Macom, Holland and Blonder-Tongue ALL have DC Pass ONLY on VHF PORT.....which would PREVENT using in the configuration most frequently recommended: UHF Preamp ONLY for UHF Antenna and NO Amplification for VHF due to likelihood of Overload/Desensitization, bearing in mind that Coax and RF Splitter is significantly LOWER for VHF Frequencies. Only other configuration of interest is to Amplify BOTH VHF and UHF, in which DC PASS doesn't make any difference. And I've NEVER run across a situation with ONLY Strong UHF Signals and very weak Hi-VHF/VHF Signals....which MIGHT actually NEED DC PASS ONLY on the VHF Port. Every source I looked at said it was Discontinued....so not more INEXPENSIVE VHF/UHF Combiners for use BEFORE Preamp......

Fol. VHF/UHF Combiners/Splitters ALL have DC PASS on UHF Port and are READILY AVAILABLE from MULTIPLE SOURCES [be sure to click on "NEW" for ALTERNATIVE SOURCES]:
https://www.amazon.com/Radioshack-Co.../dp/B001VLPQ6K
https://www.amazon.com/Antennas-Dire.../dp/B008PBTPN4
https://www.antennasdirect.com/store..._combiner.html
https://www.amazon.com/dp/B015ZSVDAW..._t1_B008PBTPN4
http://www.mcmelectronics.com/product/33-2230

I was also surprised to see RS-1236 Lo-VHF/Hi-VHF/UHF THREE Antenna Coupler on FleaBay [NO...it does NOT separate out FM from Lo-VHF...only from Hi-VHF]:
http://www.ebay.com/itm/Lot-A-Rare-3...-/122520736091

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post #219 of 226 Old 05-31-2017, 09:06 AM
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Quote:
Originally Posted by holl_ands View Post
I've NEVER run across a situation with ONLY Strong UHF Signals and very weak Hi-VHF/VHF Signals....which MIGHT actually NEED DC PASS ONLY on the VHF Port.
I have, which is why I started looking for a UVSJ, because the OP didn't want to run a second coax line and combine after the power inserter for the VHF only preamp.
http://forum.tvfool.com/showthread.php?t=16275

He has strong local UHF channels and one strong VHF channel. He has weak VHF channels in a different direction and the local VHF comes in the rear OK.

Hence, the need for a preamp for VHF only.

That is the solution described by Tower Guy, a TV station engineer in NY. I agree with the solution, but the OP isn't willing to run a second coax for the unamplified UHF antenna.

Actually, it is perfectly natural for a UVSJ to pass power on the VHF side, because the VHF low pass section has series inductors and shunt capacitors.

If a UVSJ needs to pass power on the UHF side, then a choke must be added, because the high pass UHF section uses series capacitors and shunt inductors. It also needs a blocking capacitor added on the VHF section to keep the DC power out of the VHF antenna.

If you can not measure it, you can not improve it.
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post #220 of 226 Old 06-01-2017, 02:02 PM
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IF you can't find UVSJ with DC PASS on VHF Port.....then it looks like you might have to either use ONE Preamp with VHF/UHF Combiner on Preamp's INPUT......or (eliminating ~1 dB Loss in Combiner on Input...not that 1 dB is all that important). OR, use TWO Preamps and a UHF/VHF Combiner on Preamp OUTPUTS....which means it's easy to feed DC Power to UHF PREAMP, but you'll have to use a separate DC POWER INSERTER between the VHF Preamp and the VHF/UHF Combiner....where there's a will, there's a solution. BTW: Many DistroAmps, such as CM3410, have a separate Coax Connector for Direct Connect to a DC Power Supply.
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post #221 of 226 Old 06-25-2017, 06:44 PM
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Quote:
Originally Posted by tylerSC View Post
To update this thread, it should be mentioned that MCM now sells a UVSJ from Stellar Labs, and some remaining Radioshacks still have their UVSJ in stock.
I was curious about the performance of the MCM UVSJ and compared four UVSJs for UHF insertion loss:

MCM 33-2230
Radio Shack 15-2586
Antennas Direct EU385CF-1S
Macom (Old)

For the first measurements, I used a Blonder Tongue HAVM-1UA Frequency Agile Modulator as a stable signal source, and a Sadelco DisplayMax 800 Signal Level Meter. The modulator puts out an analog signal; I used the video carrier for each channel. The Sadelco meter has 0.1 dB resolution.



I was having a problem getting consistent measurements. The Sadelco meter has a built-in calibration reference and at certain intervals will correct its readings. This was difficult for me to deal with when making 0.1 dB resolution measurements, so I switched to my Sadelco 719E meter that has a panel meter with wide 1 dB divisions in the center of the scale that allows me to interpolate to the nearest 0.1 dB.

The Sadelco 719E is on the left; the DisplayMax 800 on the right:



I used the red divisions and the second scale -10 to +20 dBmV.



I would read that as +2.8 dBmV.

To make a measurement, I first connected the modulator to the meter with an F-81 adapter and took a reading. I then substituted the UVSJ, UHF and common ports, for the adapter for the second reading.

There was an attenuator at the output of the modulator and at the input of the meter to try to keep the line "flat" (low SWR).



The MCM 33-2230 does have a higher insertion loss that might make a difference if your marginal signals are at the "Digital Cliff."

The Macom UVSJ has very low loss because it has fewer components.

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post #222 of 226 Old 06-27-2017, 09:42 PM
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So does the RadioShack version have less insertion loss than the MCM?
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post #223 of 226 Old 06-28-2017, 07:07 AM
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Quote:
Originally Posted by tylerSC View Post
So does the RadioShack version have less insertion loss than the MCM?
My measurements say YES. Extract from my previous post:

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post #224 of 226 Old 08-25-2019, 03:37 PM
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Lots of great info here, thanks to all.

I've got a really basic question. It is correct that a well designed UHF / VHF combiner will only pass the UHF from the UHF-labled input and only pass VHF from the VHF-labled input?

I'm in a situation where, because of multi-path on channel 10, I need a unique VHF antenna arrangement. I want to combine it with the signal from a separate UHF antenna (that still picks up some VHF), but I want the VHF from this feed to be ignored (or significantly attenuated)

I think a UHF / VHF combiner should do exactly this. Am I right?

... Altan

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post #225 of 226 Old 08-25-2019, 03:51 PM
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Originally Posted by MadMyers View Post
I've got a really basic question. It is correct that a well designed UHF / VHF combiner will only pass the UHF from the UHF-labled input and only pass VHF from the VHF-labled input?
If it's well designed. Correct
Antennas... location... make/models...
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post #226 of 226 Old 08-25-2019, 05:06 PM
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Originally Posted by Ratman View Post
If it's well designed. Correct
Antennas... location... make/models...
Here's the beginning of the issue started in the Atlanta section.

https://www.avsforum.com/forum/45-lo...l#post58375790

Rather than have to go back and forth to get the full set of information and responses, it'll be less confusing if we keep everything in one place.

Since this is pretty much a 2-year dormant thread, I'm closing it. All the replies are now in the Atlanta thread.

Walking the fine line between jaw-dropping and a plain ol' yawn.

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