I’ve had a few questions come my way about antenna distribution systems for AM radios, specifically to connect something like a large attic antenna to multiple radios at once. There’s a lot out there designed for VHF/UHF or CATV signals, but relatively little designed to operate way down at the AM broadcast band frequencies from 0.5 MHz to about 1.7 MHz.
After some investigation, though, there does seem to be a set of products which would do the job. DX Engineering, a popular amateur and shortwave equipment vendor, has most of the gear for sale. First, most distribution systems are expecting a specific impedance, usually 50- or 75-Ohms, so you’ll want to start with the right antenna.
First up is the AFHD-4 antenna. It’s meant to be mounted in the attic or outside and fed with RG-6 coax into the house. It’s also a good FM antenna, so it could be useful for other types of receivers than just vintage radios.
Next up, to run this out to multiple radios, you’ll need the signal splitter to provide multiple outputs. DX Engineering also offers their MBS-4 splitter, with one input to 4 outputs, covering 0.5-2400 MHz. Looks similar to a Cable TV splitter, but the frequency range goes much lower.
Finally, since most vintage radios (or even ’70s stereo receivers) don’t have 75-Ohm coax ports on the back, you’ll need a matching transformer – one of the ubiquitous 75-300 Ohm baluns which was a lot more common back in the analog television days, but still kicks around from time to time. It will convert from the 75 Ohm coax signal to a 300 Ohm leaded connection, which you can connect across your radio’s antenna and ground terminals. Reportedly, the Channel Master CM 94444 is a good choice with low insertion loss, but you may have to experiment a bit as most of these aren’t specifically measured for performance at AM frequencies. They’re cheap if you need to try a few different ones to find some that work well, though.
In addition to being easier to hook up, these matching transformers are useful for adapting the impedance to be closer to what a vintage radio will expect for better signal transfer.
Depending on your radio, and the lengths of the coax run, you might not actually need the balun/matching transformer. It does terminate the 75-Ohm coax into it’s characteristic impedance, so it works like a real transmission line and has relatively low loss, but depending on your local environment the twin leads might act as their own antennas and cause cross-talk and interference. You’ll want to experiment to find out for sure.
To put it all together in a working system, mount up the antenna in your attic or outside on your roof, then run coax to a convenient drop point and install the splitter. Now, depending on how strong your local stations are, you might not need any other equipment – but the MBS-4 splitter has 12 dB insertion loss, so you might want something to help boost the signal levels up. In that case, there’s another DX Engineering product, the InLogis LF/MF/HF receiving pre-amp. It offers 30 dB of gain on the front-end; install the amplifier between the antenna and splitter and you’ll have a great signal boosting system. If it’s too much, knock it back down a bit with 10 or 20 dB fixed attenuators to dial in just the right amount of gain you’ll need. (The amplifier system is a little spendy, at $300 with free shipping, and has a bit of a lead time on it. There are some included features which are geared more for ham radio users which drive the price up a bit.)
And of course, you might need some accessories. There’s the already-mentioned matching transformer; you might also want to pick up a spool of RG-6 coax for the new runs you’ll be making to distribute the signal to your radios. And if you don’t already have a coax crimping kit, that’s helpful. And finally, you may want some attenuator pads to fine-tune the signal level reaching your radios after the amplifier and splitter if you have trouble with the front ends overloading.
In total, if you pick up the antenna, amplifier, splitter, spool of coax cable and crimping kit, and four matching transformers you’re looking at about $600 for the full set of equipment. Sounds a little steep, but it’s a high end solution which should provide great performance and far less hassle than trying to build your own or cobble something together with parts which weren’t designed for this application.