Hello all,
I have a 450-460 TJ series combiner. I am having some slight issues in getting the TX side of the house to play nice. Cans and circulators are spot on, so I am thinking that my issues are in the critical cable lengths from the circulators to the star junction. Right now all I am doing is making tessco rich by all the cuts and re terminating that we are doing to get this thing tuned up.
With that being said, the manual states that if you order an expansion channel, that they send you 3 critical length cables and to try to tune each one to achieve lowest loss.
Does any one know the length of the three cable they send? Manual doesn't tell you. Figured I would ask here before calling Sinclair. Does anyone know a good starting point for length on this combiner?
Thanks, Rob
Sinclair TJ Series Combiner Critical Cable lengths needed
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Astro Spectra
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Re: Sinclair TJ Series Combiner Critical Cable lengths neede
You do have the circulators after the transmitters and before the cans, right?
The length of the cable needs to be 1/4 wave from the short that the off frequency signal will see at the coupling loop in the can to the centre of the spider (hub, combiner, star junction, whatever you like to call it). If there is a wide spacing then length is cut for the frequency of the other transmitter. Easy to say if there are just two transmitters, harder when there is a group of transmitters. Then you just have to take an average.
So if we have two transmitters on F1 and F2, the stub from the F1 cavity is cut to the F2 length and vice versa. Why the frequency of the other transmitter? Well because at the F2 frequency the F1 can looks like a short and we need it to be transformed to an open at the spider so it doesn't short out the F2 signal. So the 1/4 wave transformer (the stub) needs to be cut for F2. Doesn't matter what the cable length is at F1 because it should be about 50 ohms at each end.
I'm not that familiar with the Sinclair ones but I think they have right angle male connectors. The cable length needs to be measured as the ideal length less the length thru the RA male cable connector and N female connector on the can.
So if F1 is 450 MHz and F2 is 460 MHz and your cable is RG213 (VF = 0.66), then the stub for the F1 cavity to the spider is 3x10^8 / 460x10^6 x 0.66 x 0.25 = 107.6 mm. Now that's probably too short if you have 6" cans, so we go for 3/4 wave cables and the length becomes 322.8 mm. The length can be any odd number of quarter waves. That length needs to be what you measure from the top of the can to the centre of the spider, so obviously the length of RG213 is a few inches shorter. In this example the stub length for the F2 cavity is 330 mm.
Hope that helps with the Tessco bill.
The length of the cable needs to be 1/4 wave from the short that the off frequency signal will see at the coupling loop in the can to the centre of the spider (hub, combiner, star junction, whatever you like to call it). If there is a wide spacing then length is cut for the frequency of the other transmitter. Easy to say if there are just two transmitters, harder when there is a group of transmitters. Then you just have to take an average.
So if we have two transmitters on F1 and F2, the stub from the F1 cavity is cut to the F2 length and vice versa. Why the frequency of the other transmitter? Well because at the F2 frequency the F1 can looks like a short and we need it to be transformed to an open at the spider so it doesn't short out the F2 signal. So the 1/4 wave transformer (the stub) needs to be cut for F2. Doesn't matter what the cable length is at F1 because it should be about 50 ohms at each end.
I'm not that familiar with the Sinclair ones but I think they have right angle male connectors. The cable length needs to be measured as the ideal length less the length thru the RA male cable connector and N female connector on the can.
So if F1 is 450 MHz and F2 is 460 MHz and your cable is RG213 (VF = 0.66), then the stub for the F1 cavity to the spider is 3x10^8 / 460x10^6 x 0.66 x 0.25 = 107.6 mm. Now that's probably too short if you have 6" cans, so we go for 3/4 wave cables and the length becomes 322.8 mm. The length can be any odd number of quarter waves. That length needs to be what you measure from the top of the can to the centre of the spider, so obviously the length of RG213 is a few inches shorter. In this example the stub length for the F2 cavity is 330 mm.
Hope that helps with the Tessco bill.
Last edited by Astro Spectra on Mon Sep 01, 2014 7:19 pm, edited 1 time in total.
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Astro Spectra
- Posts: 668
- Joined: Sat Sep 22, 2001 4:00 pm
Re: Sinclair TJ Series Combiner Critical Cable lengths neede
Since I do this sort of stuff in the lab I thought I might show a trick of the trade. The exact location of the 'short' inside a band pass cavity using loop coupling on the input and output is not obvious and varies with the amount of coupling (rotation of the coupling loops). When doing precision matching there is this unknown and also the unknowns of the exact velocity factor and electrical lengths of the various connectors used.
We use an adjustable line section sometimes called a trombone section to vary the line length while measuring the system performance with a network analyser to get the exact cable length needed for the job at hand. Here a GR line section:
[img]http://www.surplussales.com/Images/Equi ... 001_lg.jpg[/img]
We use Narda but I couldn't find an illustrative image on-line.
We use an adjustable line section sometimes called a trombone section to vary the line length while measuring the system performance with a network analyser to get the exact cable length needed for the job at hand. Here a GR line section:
[img]http://www.surplussales.com/Images/Equi ... 001_lg.jpg[/img]
We use Narda but I couldn't find an illustrative image on-line.
Re: Sinclair TJ Series Combiner Critical Cable lengths neede
Why are you paying Tessco to build cables? The tools and parts aren't that expensive... learn how and do it yourself.FMROB wrote:Right now all I am doing is making tessco rich by all the cuts and re terminating that we are doing to get this thing tuned up.
Re: Sinclair TJ Series Combiner Critical Cable lengths neede
Why not just order in what you need from Sinclair? I have found them to be very helpful and reasonably priced.
Re: Sinclair TJ Series Combiner Critical Cable lengths neede
Thank you all for the replies!!!! there is a lot of info here, so I am going to print it out and go over it tomorrow. We are using 6 cans with a 6 way star junction. It is a mix of 450 & 460 channels to help complicate everything.
We have enough equipment to be dangerous, tracking gens, analyzers, watt meters, etc. We do build any cables we need ourselves in house. I was referring to the pile of new connectors that we terminated and cut to try to find the "right length". Tessco made out on us this week.
I was going to give Sinclair a call, but we wanted to try to vet out the problems ourselves as a learning experience. We got 4 cans tuned up nicely, and was only losing with a test 30 watts in, 18 watts out thru the system. When we added a 5th can, it tanked the entire system and that's where are problems arose.
We haven't played with combiners all that much, so it is new ground. After we build this, I have a 8 channel telewave 450-8TRM that we have to get on channel, not looking to be too much fun.
More to follow,
Thanks, Rob
We have enough equipment to be dangerous, tracking gens, analyzers, watt meters, etc. We do build any cables we need ourselves in house. I was referring to the pile of new connectors that we terminated and cut to try to find the "right length". Tessco made out on us this week.
I was going to give Sinclair a call, but we wanted to try to vet out the problems ourselves as a learning experience. We got 4 cans tuned up nicely, and was only losing with a test 30 watts in, 18 watts out thru the system. When we added a 5th can, it tanked the entire system and that's where are problems arose.
We haven't played with combiners all that much, so it is new ground. After we build this, I have a 8 channel telewave 450-8TRM that we have to get on channel, not looking to be too much fun.
More to follow,
Thanks, Rob