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Looks like the L7A is the bridge. Or no? https://scontent-atl3-1.xx.fbcdn.net...fc&oe=5F0899E6 |
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It appears L7 is meant to move to L7a location for the 0264 version. Your preamp is probably newer. . |
The "Less Loss" harness begins...
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Wow ! OK, I have to say I am pretty fired up! My ADD (Antenna Distraction Disorder) is kicking in. You have made it very clear for me- a novice. This will help others too I'm sure, if they take the time to read it. Oh, the twists in the twin Lead, Sev was telling me, it's up to me how many twists, just make sure they are the same amount per side. That may be the trickiest part. |
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Seems like I always pick something complicated to do and it's hard to get input that way. So, I really appreciate it. |
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Bare copper vs coated copper. It also would appear they tuned the G1 next to the trap adjustment. It has a silicon strip across it to hold the coils in place. If somebody had a CNC machine to mill the traces. That board would be pretty easy to replicate after inputting the necessary data. Then it would be a matter determining the ferrite core. Or install half wave loops and sourcing the rest of the components. It would be a nice challenge for a hobbyist. The units have 3.0 and 2.2 dB in noise. I wonder if it would be possible to determine and optimize the components creating the noise. Jeff Kitz has shown that it is possible to create low noise preamp. Then there is Winegards LNA-100. Also very low noise. I would imagine a portion of the noise is being generated by the PSU. The 3 capacitors @ C7, C13 and C28 would be suspect as well. |
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"OK, I understand about the ferrite to form a choke balun around the coax, but there are two questions I have in mind: 1. If you connect two 300 twinleads together in parallel, that would give you 150 ohms. Wouldn't that be a 2:1 missmatch with the 75 ohm coax? 2. When you connect the two twinleads in parallel, they must be in phase. It's like connecting two batteries in parallel: + to + and - to -. How will you know which way is correct?It will work, but there will be some loss because of the mismatch which causes an SWR of 2:1. It is difficult for me to predict how much additional gain two antennas connected that way will give you compared to just one antenna. There are two problems to be concerned about when you connect together in parallel the two 300 ohm lines from two antennas. The first problem is the mismatch loss because two 300 ohm lines in parallel will give you 150 ohms, which you will then connect to a 75 ohm coax line. You aren't going to like to hear this, but you would need a 1/4 wave matching section of 106 ohms between the 150 ohm point and the 75 ohm coax for a perfect match. The second problem is getting the two 300 ohms lines in phase (or polarity if you prefer) when they are connected together in parallel. You have a 50/50 chance of getting it right. If they are connected out of phase the main lobe will split in two like this" https://imageevent.com/holl_ands/sta...f5v5sec1.goose Ooops... looks like I have another problem to solve? Now that we're in the 11th hour I'm so glad that I did not execute my plan fully. I'm kind of glad it was chilly Friday after work and I had to do some deck work around the house. That kept me off the roof! Anyhow what should I do? What do I do to achieve the correct match? |
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Channel Master and some other manufacturers have not provided accurate noise figure numbers. Numbers often provided are quotes from transistor manufacturers specs. There is more to noise figure than transistor specs, for example: - The excellent input filtering of the 0264/0064 and other older CM preamps, contribute to the noise figure. - The loss in the ferrite balun adds to the noise figure. Quote:
The first UHF transistor is a BFG67 (1st transistor is usually main active source of noise) ------------------------- The 0064 & 0264 are good and reliable preamplifiers. The design, except for input baluns, is virtually the same as for the old CM7778. The old CM7778 was cloned to the RCA_TVPRAMP1R. http://forum.tvfool.com/showpost.php...76&postcount=2 . |
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I haven't tried it in situations like this thread is addressing, but I suspect it might be worth experimenting to lower noise. One advantage of non-polarized CAPS is that is doesn't matter which way you solder them in. Also, they're not too expensive as compared to regular CAPS and can still be bought at places like Mouser Electronics: https://www.mouser.com/Search/Refine...zed+capacitors You Techs here are above my pay grade, but I hope this might be of some help. |
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This image popped up on google. A thread with you and Pete Higgins. https://forum.tvfool.com/showthread.php?t=13530 Do you know of any alternative componets that would lower the noise figure. Or based on the design it is baked in? https://forum.tvfool.com/attachment....7&d=1376151612 One of the members a long time ago mentioned to me that the RCA was a clone of the 7778. The first preamp I picked up actually. So based on Calveras's testing. The CM's and RCA's had a noise figure very similar to the Blonder Tongue Galaxy III's. Assuming that B.T. was being truthful about their noise levels. I have come across a few B.T. 300ohm dual inputs with the 75ohm output but passed no them because there did not appear to be much love for them due to their advertised noise figures. Any idea if those figures are actually accurate or not? To clarify a statement back on post #14 you stated you removed the ferrite bead and replaced it with a half-wave loop on a Spartan? OTAFAN posted an interesting idea concerning replacing the polarized caps with non polarized. Thoughts? Modding the board should not be very difficult. Though I would like to find another Spartan to test on. |
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Interesting idea. Need more data on it to see what the possibility is. I knew guys years ago that were audiophiles. They were just as bad if not worse than the car guys trying to squeak out every very last bit of HP out of their engines. |
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It might lower noise figure a couple of tenths of a dB. Prrobably need to retune. The transistor substitution works fine, did that when replacing balun. Quote:
The older 0064/0264 have about the same noise figure as the BT Galaxy series. Quote:
http://forum.tvfool.com/attachment.p...2&d=1592234723 The coax is RG-180 (95 Ohm). The impedance is not ideal, but in some ways better than 75 Ohm coax. It could be better if the impedance for the loop was 150 Ohms. But, such cable would have to be custom made and it would probably be too large to fit in the case. Quote:
Some audio circuit paths require passage of bipolar signals (plus & minus). Audio signals are very low frequency relative to RF (long wavelength, calculate wavelength for say 20 Hertz), therefore large capacitors are needed. All bipolar signals in an RF amplifier pass through small ceramic capacitors, they are essentially nonpolar. The polarized capacitors in RF amplifiers are for filtering the DC (not bipolar) supply voltages. The noise that is bothersome in preamplifiers is usually in the same frequency range as the desired signals (MHz). . |
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I assume that 150ohm Belden Twinax would not be appropriate for use for the half loop? |
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It probably could be made to work for a half wave loop balun. but would require some unconventional considerations. In general, coax is better for the half wave loop. --------------- Referring to the 0064 with half-wave balun in previous image. The input & output of VHF portion was disconnected and powered down. There is some noise figure advantage in not having a VHF circuit connected to a UHF circuit. Anyhow, that balun would not work well for VHF. --------------- FYI, will not be logging in frequently for a while. . |
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Hope all is well on your end. |
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