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Old 2009-02-03, 12:26 PM   #61
cerberii
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what is A? A=network output/network input or viceversa depending on your sign choosing
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Old 2009-02-04, 03:41 AM   #62
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Sketchy parts descriptions masquerading as real specs are worse than no specs at all....
What are the conditions for 0.7 dB insertion loss?
Minimum they ever measured for the best hand picked part at the best frequency???
Guaranteed max??? 99.99 percentile??? And my favorite: "typical".

Minicircuits has an extensive line of 4:1 Baluns. Fol. is typical 1.24 dB loss at 700 MHz:
http://www.minicircuits.com/pdfs/TCM4-19.pdf
As America Ferrera once said, "Real specs have loss curves."

They also carry 1:1 Baluns, such as fol. with half as much loss:
http://www.minicircuits.com/pdfs/TC1-1-13M-75+.pdf

Since their on-line search engine is very difficult to use, suggest ordering their catalog.

And here's an article re an even lower insertion loss Anaren Multilayer 4:1 Balun:
http://www.mwrf.com/Articles/Print.cfm?ArticleID=15084
See Fig 4 for VHF/UHF band loss. Call for more info, I didn't find it on their website.
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Old 2009-02-04, 03:30 PM   #63
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has anyone used these PCB baluns which holl ands linked ?
the one I clicked on had a minimum order of 100

I'm interested, but how do you go from the product shown to something you can use on your CM4228: you must need to solder wires onto it at the very least ?
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Old 2009-02-04, 05:36 PM   #64
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Quote:
Sketchy parts descriptions masquerading as real specs are worse than no specs at all....
What are the conditions for 0.7 dB insertion loss?
Minimum they ever measured for the best hand picked part at the best frequency???
Guaranteed max??? 99.99 percentile??? And my favorite: "typical".
Heh, heh. "HDTV ready"
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Old 2009-02-05, 12:34 PM   #65
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For anyone contemplating using a direct coax connection to an antenna....

When coax is directly connected to a balanced antenna, a series of
choke ferrites can minimize perturbations to the antenna pattern....
they ALSO reduce pickup of locally generated noise along the coax:
http://www.w8ji.com/common-mode_noise.htm
This is probably more of a problem at VHF than UHF....

Choice of Ferrite material must be appropriate for operating freq, which
explains why they are popular for narrow band Ham radio bands but
NOT so much for wide VHF & UHF TV bands. Analysis is also difficult:
http://w2du.com/r2ch21.pdf
http://www.audiosystemsgroup.com/RFI-Ham.pdf
http://www.vk1od.net/balun/G1-1-FT140-61/
[The latter is a current balun that wraps coax around a Ferrite core.]

Ferrite Core kits are usually available from Ham Radio enthusiasts:
http://www.mouser.com/catalog/630/747.pdf
http://www.minikits.com.au/doc/section7.pdf
Hi-VHF TV Band is closest to 144 MHz Ham band.
UHF TV Band is between 420 MHz and 920 MHz Ham bands.
http://www.arrl.org/FandES/field/reg...ands_color.pdf
Where you will see that it takes MULTIPLE Ferrite choke cores
on the coax to provide the desired amount of impedance.

On-line catalog and an earlier *.pdf file can be found here:
http://www.fair-rite.com/newfair/catalog.htm
http://www.fair-rite.com/newfair/pdf...te_Catalog.pdf

Or use one of the several quarter & half wavelength Baluns:
http://www.iw5edi.com/ham-radio/files/I0QM_BALUN.PDF
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Old 2009-02-05, 09:42 PM   #66
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Quote:
Originally Posted by holl_ands
Or use one of the several quarter & half wavelength Baluns:
http://www.iw5edi.com/ham-radio/files/I0QM_BALUN.PDF
Cool! I like that link, because it confirms what my mind had worked out about a nifty H-frame matching circuit I saw once.

That design dispensed with the extra 1/2L section of coax, and just connected the two 1/4L impedance match sections directly to the center feed lines of the H-array, but 1/2L apart, getting the same end result for free.

The scary part is, I think I fully understood it.

So now, for my own 50ohm quad-dipole H-array, I just need to fabricate a pair of 1/4L matching sections with a 61ohm impedance (to connect the 50ohm H-array to 75ohm RG6).

Thanks!
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Old 2009-02-06, 05:56 PM   #67
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Perhaps K6STI can investigate effect of using other than the "calculated" coax impedance
(e.g. standard 50 & 75-ohm)....I'm going to guess it isn't going to be all that important,
given the low-Q, broadband nature of the beast...
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Old 2009-02-06, 08:29 PM   #68
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I looked at the I0QM 1:1 design. The voltages at the load are very different and the mismatch bandwidth is not that great. It may have an application, but I doubt it is suitable for UHF TV.

A friend just modeled a quarterwave 75-ohm line in series with a quarterwave 90-ohm line, simulating the sudden change in trace width and characteristic impedance of the UHF-TV printed circuit balun whose image I posted. It turns out to match about 250 ohms instead of 300 ohms. I tried reversing the order of the lines and it matched 350 ohms! This not-exactly-4:1 impedance transformation could be quite useful.

For a 4:1, the optimal impedance is 150 ohms, which could be implemented on a PC board. The resulting mismatch loss is negligible over the entire UHF-TV band.

Brian
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Old 2009-02-06, 11:13 PM   #69
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Quote:
Perhaps K6STI can investigate effect of using other than the "calculated" coax impedance
Wait a sec.. are you the same K6STI of YO fame?

Zowie!, this forum attracts some interesting people!

Cheers!
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Old 2009-02-07, 01:22 PM   #70
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Indeed it does. Awhile back, thanks to his nuts-and-bolts analysis of a particular FM Radio tuner, I bought one and concur with his review.
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Old 2009-02-07, 06:27 PM   #71
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Quote:
Originally Posted by 300ohm View Post
At that price range, just get a CM0064 or CM0264 300ohm in 75ohm out preamp and kill a couple of birds with one throw.
Doing so just pushes the problem elsewhere. The CM Spartan 3 models
are fundamentally a 75-ohm input design, so we don't sidestep balun
loss. Model versions with a 300-ohm input(s) have 4:1 internal baluns
to match the incoming line impedance. Actually IIRC every 300-input
preamp I've ever seen has had an input balun.

Nothing fundamentally wrong with doing so other than having to deal
with 300-ohm twin lead -- if you can still find it around these days.
And there is the path routing consideration for twin lead of maintaining
a distance between it and metallic objects. Not that we're going any
appreciable distance with twin lead but it is a bit of a pain to deal
with and in terms of durability it just doesn't have the physical
ruggedness of coax which is a maintenance issue.
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Old 2009-02-07, 06:39 PM   #72
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Quote:
Originally Posted by k6sti View Post
My power supply / injector is in the garage for the same reason. It drives the preamp with AC power. That really threw me when I first opened the preamp and poked around. Any idea why they did that rather than just feeding it DC?
Designing the amp for AC power feed removes the
possibility of a polarity reversal mishap and allows
it to be powered from an arbitrary AC or DC source.
DC line fed preamps ideally should have some sort of
internal polarity protection so this is just a generalization
of that need.
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Old 2009-02-08, 01:32 AM   #73
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Quote:
Doing so just pushes the problem elsewhere. The CM Spartan 3 models
are fundamentally a 75-ohm input design, so we don't sidestep balun
loss. Model versions with a 300-ohm input(s) have 4:1 internal baluns
to match the incoming line impedance. Actually IIRC every 300-input
preamp I've ever seen has had an input balun.
True, theres a 300 ohm to something matching network in there. But at least its a Channel Master build quality internal matching network. That accounts for some positive in my book.
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Old 2009-02-08, 10:52 AM   #74
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Quote:
But at least its a Channel Master build quality internal matching network. That accounts for some positive in my book.
Not to mention all of those low-loss directly soldered connections, and the possible use of a 50ohm internal preamp with a corresponding non 4:1 balun to match it.
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Old 2009-02-09, 06:44 AM   #75
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The C-M Spartan picture shows a perfectly normal 4:1 balun.
Non 4:1 baluns are fairly complicated....
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