Combining Stacked UHF-TV Arrays

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k6sti
Posts: 63
Joined: 20 Oct 2014 17:27

Combining Stacked UHF-TV Arrays

Post by k6sti » 25 Nov 2015 13:43

There has been a lot of nonsense posted on blogs and other forums about combining the signals from stacked UHF-TV arrays. I decided to analyze a number of effective methods, some rather unconventional. If anyone knows of another good method, please tell me and I'll include it. The writeup covers the U.S. 470-698 MHz UHF-TV band, but the principles apply at any frequency.

Scroll down to the end here:

http://ham-radio.com/k6sti/tv.htm

Brian

daveB
Posts: 1144
Joined: 07 Oct 2014 16:49

Re: Combining Stacked UHF-TV Arrays

Post by daveB » 25 Nov 2015 18:23

I had 4 x Triax BB grids fed into a 4 way Triax power combiner feeding a low noise pre-amp miunted on a rotator at about 23 ft. This was back around 1990 when it was all analogue. That certainly pulled in the DX! The power combiner - if I remember correctly - had a loss of around 0.2dB (but it IS 25 years ago. At that time Triax actually produced a full kit for this installation which cost me around £100.

I was also very keen to note that you have published a design for the small yagi for Band 3 (High-VHF). Unfortunately in Europe the band is 174 - 240 MHz with the bulk of the transmitters in the UK between roughly 216 and 230 MHz.It is only recently that they have started to use lower frequencies to expand the service. I've tried to model the antenna on 4nec2 as an excercise using 45% as the factor from the band 2 design So far have only managed around 16 db f/b ratio but directivity is very similar to the Band 2 version and SWR is a max 2.1 at the ends of the band.

David
SDRPlay, AirSpyMini, RTL dongles, SB920s, Yamaha TX930. 5 ele compact yagi on rotator at 18ft agl. FM5 facing east, FM5 facing south, FM3 facing NE, OIRT dipole, 3-ele Moxon vertical facing east.

k6sti
Posts: 63
Joined: 20 Oct 2014 17:27

Re: Combining Stacked UHF-TV Arrays

Post by k6sti » 25 Nov 2015 18:46

David, that is very low loss for a power combiner if it is at UHF. Perhaps it was the parallel kind plus a quarterwave coaxial transformer, like the ones hams use for transmitting. At 98 MHz, I measured 0.3 to 0.35 dB loss for my junk box hybrid splitters with one at 0.5 dB. The loss goes up quite a bit at UHF.

Here is a design for 174-240 MHz:

http://ham-radio.com/k6sti/dab.htm

Brian

pe1etr
Posts: 1244
Joined: 07 Oct 2014 00:40
Location: Blackpool UK

Re: Combining Stacked UHF-TV Arrays

Post by pe1etr » 25 Nov 2015 19:29

Hi Brian and Dave,

I remember these combiners. They were known as "Stripline Combining Filters", and in addition to Triax (Denmark) a USA company called "Lindsey Electronics also made a similar device with a different name. They have insertion losses of 0.2 - 0.3 dB.
I remember that "Resistive splitters/combiners" had losses of about 3dB (so all of the gain from stacking two aerials was lost), and there was another type called "Ferrite core combiners (I think), which is probably the equivalent to your "Hybrid Splitters".

A loss of 0.3 dB - 0.5 dB would have been considered very good indeed back in the 1980s /90s.

Peter Wilson, Blackpool UK

Andrew Webster,Wigan
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Joined: 18 Sep 2014 16:03
Location: Wigan Northwest England
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Re: Combining Stacked UHF-TV Arrays

Post by Andrew Webster,Wigan » 25 Nov 2015 21:06

I stacked two Wolsey colour kings with a Labgear passive splitter and the loss was about 3 db that was the ferrite type but I didn't notice it any better than a single Colour king so I abandoned the idea and bought a Triax 92.

I put the colour kings one above the other with separate feeders on a mast 10 ft high at the back of the house for French reception.

Andy
Receiving equipment: 2 Sony ST-S311, 1 Sony ST-SE520, 1 Sony ST-SE700 2 crossed FM 5s rotatable, 1 homemade FM 3 horizontal beaming S/E, 1 vertical FM 3 fixed beaming at Ireland, 2 element band 1.

k6sti
Posts: 63
Joined: 20 Oct 2014 17:27

Re: Combining Stacked UHF-TV Arrays

Post by k6sti » 25 Nov 2015 21:37

pe1etr wrote:...there was another type called "Ferrite core combiners (I think), which is probably the equivalent to your "Hybrid Splitters".
Yes, a hybrid combiner/splitter uses a wideband ferrite transformer and a balancing resistor. The resistor dissipates power only when the loads are unequal. The advantage of a hybrid splitter is that the output ports are isolated. So one TV local oscillator won't get into a second TV hooked to the other output port, for example. When used as an antenna combiner, the isolation provides no advantage that I know of. I use these devices because I find them at garage sales for free or next to nothing. The other kind of combiner/splitter mentioned, where the center conductors are joined and fed to a quarter wavelength transformer to restore the proper impedance, has less loss, but I never see them at garage sales (!). I analyzed a quarterwave transformer and SWR was 1.24 or less across the entire U.S. TV band of 470-698 MHz. I was surprised it worked so well over such a wide bandwidth since it's a quarter wavelength long only at the center of the band. An SWR of 1.24 yields a mismatch loss of 0.05 dB, which is negligible.

Brian

k6sti
Posts: 63
Joined: 20 Oct 2014 17:27

Re: Combining Stacked UHF-TV Arrays

Post by k6sti » 05 Dec 2015 10:02

I found one more method for combining 300-ohm antennas. It does not require custom phasing line, power combiners, or baluns. Simply join two antennas with equal lengths of 300-ohm twin-lead. Then match the resulting 150 ohms to 75 ohms with a 107-ohm transformer formed from side-by-side quarter wavelengths of RG-58 coax. Use a ferrite bead on the 75-ohm feedline. You can eliminate the quarterwave matching section and feed the junction directly if you're willing to accept the resulting SWR of 2, which yields a mismatch loss of 0.5 dB. I doubt you'd notice any degradation from this loss on an analog signal. It would affect a digital signal only when it is very close to the decoding threshold.

Brian

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