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1 a. This would reduce the installation cost of the enclosed, shielded, mast-mount, powered remote AM antenna I'm saving up for. If I can feed it into the existing and new FM cabling and tap off into the AM RCVRs in the house.
1 b. What should I use to tap into the AM signal, off the wall plates? A splitter and then? a balun and a bit of twin ribbon?
2 Why does Timbo WANT to do this?
a. Any AM antenna inside gives lots of crackles. Even tunable coils. And, there are/will be three other AM/FM rcvr s in the house.
b. Several AM stations here do music of value to me, some of the time, and most of them transmit with a wide audio BW to boot.
2c. I have a wide audio BW AM rcvr, mono only, but ICGAS. And there are other AM/FM boxes in-house
2d. News! I am becoming irritated by the breathless, agitated, style of 24hr news on TV. Particularly the 'morning presenter genre'. Our ABC (sort of BBC?) radio news manages to avoid this.
TIA
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Follow Ups:
The supplier has come up with a solution that will do what I want.
I'll spell it all out later.
I will need to do the cable run install and the wall plates, but I can manage that.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Just to be clear, you're not wanting to carry both FM and AM on the same coax, correct?
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Buy Chinese. Bury freedom.
If it can work it would save me quite a bit of money.
After all my previous network had DTV AND FM coming down the same coax network, split at the wall plates which had TV and FM outlets and some electronic components behind the plates.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
It seems to me there should be a commercial solution for this, but I haven't looked. Let me just throw out a few thoughts.
The most significant obstacle is that a balanced FM antenna requires a balun to drive unbalanced 75 ohm coax. Most commercial baluns are designed to work over the range of FM and TV frequencies. The lowest frequency of interest is therefore mid-50 MHz, so the balun - essentially a transformer - will have an inductive reactance suitable for that lower frequency limit.
Regardless of the type of AM antenna in use (balanced or unbalanced), the output cannot be connected directly across either side of the FM balun. At AM frequencies, the inductive reactance of the balun is virtually a short circuit. So, even though AM and TV signals can be carried simultaneously on the coax, they must be combined in a way that provides isolation.
One technique I can envision, just off the top of my head, would be to add a small coupling capacitor in series at the output of the FM balun. The AM antenna (either inherently unbalanced or using a balun) could then be connected to the coax. The coupling cap would present a high impedance looking back into the FM balun, so AM signal loss would be minimized. Likewise, it might be necessary to isolate the FM signals from the AM antenna/balun. This is because the internal capacitance and/or self resonance of a low frequency antenna (such as a loop) or low frequency balun can degrade the FM/TV signal strength. The simplest solution would be a small series choke to keep high frequencies out of the low frequency network.
Unfortunately, all of this only addresses one side of the issue. In order to work properly, isolation techniques need to be used at both ends of the coax. The coax carrying the AM signals cannot be fed directly to a VHF/UHF transformer-type coaxial splitter. Nor can it be fed directly to the input of a TV or FM tuner if an internal high frequency balun is connected to that point. In either case, the AM signal will be effectively shunted to ground throughout the system. Standing waves will also be created on AM frequencies, but the effect of that is a topic for a different day. :)
Perhaps there's a more practical means to accomplish this, but it hasn't occurred to me as of this writing.
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Buy Chinese. Bury freedom.
Hi Timbo,
Interesting project. What you plan to do seems OK - however, as you probably know, you should try to match impedances on both ends to the 75 ohm coax as close as possible. Suggest you do a web search on "homebrew remote (or mast mounted) low frequency preamps" and see what you find. I'd certainly try a 300ohm balun on the receiver side. Of course the other signals on the cable will take a sensitivity hit but hopefully that will be un-noticeable.
Charles
O yeah, what does the manufacturer of the powered antenna you plan to buy recommend?? Must have a "receiver side" power box - how does that "send out" the signal to a receiver? - maybe all the work is done inside it!!!
I suspect that your household coax is RG6 category. RG6 is an improvement over RG59 that can handle higher frequency signals. Carrying "medium wave" AM signals should be "a piece of cake". A huge rub lies in the interface between coax and receivers. "Typical" AM receivers either completely lack a connection for an external antenna or are configured for attachment to a length of wire. In addition, there is the matter of ground loops. Coax is an "unbalanced" configuration, with a grounded shield. IMO, the last thing you want is cabling cross connecting the chassis of several devices in different rooms.
An item for study regarding the mechanical interfacing of receivers to coax is the Motorola connector found in automobile units. Unfortunately, you are left with issues of distribution and isolation. Perhaps opto-isolators could be employed. RF transformers that work with AM signals are comparatively large and will be expensive.
Eli D.
I just checked.So, I will try a splitter into a 75/300 balun for AM, and enough twin fig-8 cable to reach the two terminals.
[The old network was installed by me along with the antenna mast and antennas, over the years. It all used to be RG11, and RF TV and FM were shared on that network. The two spare half used reels came from an office IT network installation. RG11 is a real bastard to install through hardwood base-boards. Working under a sprung hardwood floor, real close to the ground, plus left-over tiles, timber, rubble, timber, and spilled mortar, let alone spiders et al, isn't heaps of fun and I'm getting old.]
Now it's all RG6, separate runs for TV and for FM. from wall-plate to wall plate.
That is, no splitter-baluns under the roof tiles OR under the house. I'd probably need some kind of matching connection in-to the FM coax, anyway.
OTOHand, an alternative to all this speculation is to tell the Australian MFRs of the mast-mount AM antenna what I want to do and get their advice.
I'll try that mid-next week.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 08/16/19
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