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Russian VHF Antenna Diplexer Design and how it impacts ignit

 
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mark.bitterlich(at)navy.m
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PostPosted: Fri Jul 13, 2018 1:07 pm    Post subject: Russian VHF Antenna Diplexer Design and how it impacts ignit Reply with quote

Rob,

I have changed the subject line, since the topic is not IAW the original.

Concur on all observations and comments that you made.

Yes, I can locate and sweep the original Russian antenna, after I manage to gen up some connectors and so forth, but in the end what is the real sense? We already know the intent of the design. The premise is that their design was shunting low frequency energy thus possibly reducing the amount of hash that was being fed into the VHF's radio first stage RF amplifier. I hate re-creating the wheel, (unless I am being paid to do so) and I doubt you really want to either, especially if there happens to be a cheap alternative, and there is. "Cheap" being a subjective term I admit.

The Comet and Diamond Antenna sites contain specs and alternatives for an experiment like this. One has a male PL-259 pig-tail input with two SO-239's coming out of the Diplexer for A/B Port outputs. So there are a few options when it comes to how to interconnect whatever you choose.

We are looking at a 45-50 dB reduction of hash in the frequency range of 1-30 Mhz or so, (and for the uninitiated that's 10 to the 5th power) and that is where I believe the majority of noise is going to be (we agree again on that one). I have seen prices as low as $60 (U.S. Dollars) for something like this, and a 50 ohm load is simple to construct for the low freq. port.

Of note is the fact that I have used these devices myself, (hence how I know about them) mainly with some ham radio equipment. They work extremely well and meet specifications. So 60-70 dollars will allow you to put one of these in line and see if it does indeed reduce spark plug noise. If it does, you've come up with an idea that will impact the whole community, since a lot of us use Dennis's auto spark plug conversion, which on occasion has been known to increase spark plug noise just a tad. It's a great idea, and the premise has basis in theory and factual experience.

Mark

________________________________
From: owner-yak-list-server(at)matronics.com [owner-yak-list-server(at)matronics.com] on behalf of Rob Rowe [yak-list(at)robrowe.plus.com]
Sent: Friday, July 13, 2018 8:57 AM
To: yak-list(at)matronics.com
Subject: [Non-DoD Source] Re: Circuit breaker size for pitot heat?



Mark,

Thanks for your useful insight as always ... I don't have an original antenna to inspect, short of stripping the one out of my aircraft, and neither the analyser kit even if I did. So if you have an original antenna to hand and an analyser that would be very helpful to better understand empirically what is actually going on.

As you point out this was designed as a 'splitter' and not for the purpose of reducing RF interference into the original Baklan radios. But from practical experience (identical kit installs, by 40+ year avionics guy with same loom design and materials) it does appear to help some newer 8.33k replacement radios that are more sensitive to RFI.

I agree from Fourier principles the square wave RFI 'hash' will run across the band at odd harmonics that rapidly diminish in magnitude with increasing frequency, hence the high pass takes most of the lower frequency 'energy' out that probably helps the RF amp from being overwhelmed by transients. The original Baklan radios may not have needed this, but replacement radios seem to find it beneficial.

The attached schematic (extracted from a Russian doc) shows how simple the circuit is, that relies on the RF cable impedance to provide the 'R' component of the filters. Annoyingly no component values are given, nor on the original schematics, where it just refers to it as being factory manufactured, so custom made.

Given this is only a first order filter the roll-off slope is gentle, and to avoid attenuating the aircraft comms band, the Fc might be as low as 30Mhz. It will be interesting to see what your analyser results yield.

Hence finding a higher-order in-line diplexer alternative, as you've come up with, could be a big help and as I don't have the kit, or radio expertise, to provide definitive answers I really value your skills here.

Rob


Read this topic online here:

http://forums.matronics.com/viewtopic.php?p=481632#481632


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http://forums.matronics.com//files/baklan_antenna_filter_385.jpg


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Rob Rowe



Joined: 10 Jan 2006
Posts: 124
Location: Berkshire, UK

PostPosted: Sun Jul 15, 2018 7:27 am    Post subject: Re: Russian VHF Antenna Diplexer Design and how it impacts i Reply with quote

Mark,

Your time, your call ... no issue with that from me. For US products this side of the pond read the GBP price as being numerically the same as USD.

Hence I'll take a back seat on this one too as my aircraft not impacted ... anyone with an RFI problem can pick this up from the archive if they want to try it out ... thanks for renaming the thread accordingly.

While on the topic of RFI ... for those of us with Trig TY91 / TY92 comms boxes the factory have recently updated the firmware to v1.10, that apparently improves RFI immunity and (separately) in-built intercom VOX cockpit noise rejection too.

Tried this update on my -52 and, while I cannot vouch for RFI improvement, I found the intercom VOX performance to be noticeably better ... YMMV.

Rob R.


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