Any idea the best way to couple a ferrite loopstick antenna to a car radio input for AM reception?
I experimeted using a 6 inch loopstick (230uH) and 365pF parallel tunning cap from antenna input to ground (chassis) with reasonable results, but I'm wondering what the best approach is.
The tuning is very broad and I don't get much of a peak when adjusting the capacitor, but all the local stations come in.
IIRC the normal approach if you wish to make it tuneable is to have a large winding which is tuned with the cap, and a smaller winding with less turns, so there is an impedance transformation.
I suspect very much depends on the device used as the front end - whether it is a high Z fet or a lower impedance device.
It sounds like the Q is low if the tuning is broad, but do you really want the hassle of returning the capacitor every time?
I read in sci.electronics.design that snipped-for-privacy@att.net wrote (in ) about 'Ferrite bar antenna for car radio?', on Tue, 13 Sep 2005:
You need to tap down the coil or wind a secondary winding on the rod. The car radio antenna input is a lowish impedance, not 50 ohms or anything like, but much lower than the resonant impedance of your tuned circuit. But if you tap down too far, you will need to retune the loopstick for each station.
Experiment with taps or secondary windings. Alternatively, you could put a larger fixed cap in series with your 365 pF at the earthy end, and connect across it to the antenna socket. You might try a number of values from 1000 pF upwards.
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Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
Why bother with "best", unless you mean "easiest". There is no sense in trying to improve sensitivity beyond the point where you can hear the background RF noise. And on the AM band there is a lot of it. The radio itself is probably already doing as well as it can using selectivity to limit background noise. So if you connect the loopstick antenna any way at all that produces enough signal to hear the background hiss, then that is good enough, and no further overall improvement can be had by improved impedance matching or tuning. To check if your connection is adequate, just turn up the volume until you hear a good deal of hiss, while tuned to a frequency where there is no local station. Then disconnect the antenna completely. If the hiss drops to almost nothing, then you have as good an antenna as is possible. But if the hiss remains the same or drops only a little, then that hiss is coming from the radio itself, and a better antenna match could improve things.
Hard to guess from the distance. Most likely your ferrite antenna is fine but the impedance of the radio is a bit low. That loads down the LC circuit formed by your ferrite coil and capacitor, reducing its Q and thus making the tuning "broad".
I am afraid you need a FET follower at the antenna, to provide a Hi-Z to the ferrite antenna and a lower Z to the cable that goes into the radio.
I think Bill lives in the US. Out here AM is very much alive and kicking. While on the long drive to the airport I listen to it all the time. Same when doing some "real" hardware in the shop.
Car radios used to be designed for a 93 ohm input impedance, and already has a tuned front end. Unless there is something to buffer the tuned antenna, you will detune the existing front end circuit. The whip "Antenna is really a RF probe fed to the top end of the tuned circuit. Good designs used a variable cap in the circuit to null out the cable capacitance's effects on the tuned circuit. You tuned the radio to a station near 1300 KHz and adjusted it for peak reception. I could scan the input circuit from one of the old Sams AR series car radio manuals and post it on ABSE if anyone wants to see it.
Doesn't your normal car antenna do this already? It does on my car radio.
Most cars since around 1992 use diversity antenna's which work better for AM reception. If your car has a rear window heater it may also double as an AM antenna.
I would also think that trying to use an add-on ferrite loopstick antenna would allow pick-up of all sorts of local interference coming from car ignitions etc. And it would also be directional thus making sensitivity dependant on orientation to the transmitter while driving.
The FM band has whole MHz worth of bandwidth hi-jacked by the BBC so as to provide nationwide coverage but it strikes me as over-kill. Each BBC FM service uses about 2 MHz. I can often pick up one of their stations on 3 different frequencies. E.g. I can get Radio 4 on 93, 93.2 and 93.5 MHz.
Stupid regulations intended to ensure breadth of programming simply mean that most commercial stations are virtual clones of each other with bland look-alike programming. In a similar vein 'narrow-casting' is discouraged, the only 2 stations offering a unique and specialised programming style being Jazz FM and Classic FM. There's no 'rock channel' at all ! Oh - and BBC's Radio 3 already offers a full classical music service anyway.
No shortage of phone-ins to let the local retards spout off about rubbish though !
BBC's Radio 1 which is their 'contemporary music' channel plays the kind of stuff that you'd expect to hear at the kids' dance raves mainly ( and almost exclusively - to the extent that a spokesman for that channel once stated that they weren't intereted in an audience aged over 25 ? ) whilst their Radio 2 offers MOR for oldies that's 80s weighted - Yuk.
Sad. Maybe it can be revived with some local stations.
FM is bland here as well. IMHO that is because of conglomeration, large companies owning a whole slew of stations and "adjusting" the music to mainstream, the same songs over and over. But the local ones is where you can occasionally find the real blue grass or old style country. AM is a bit better, lots of ethnic broadcasts but don't count on understanding the languages.
AM talk radio is popular here, mostly conservative. Some of it has pretty good quality content so I listen to that once in a while. Surprisingly most of the contractors seem to listen to it as well when on a remote job. Also, the driving distances are huge out here. When you are going clear across a place like Nevada AM is the only thing that really works in many areas. Actually, when I drove in north eastern Scotland in the 80's it was similar. Not many FM stations but this was in an old Ford Cortina with a weak radio.
I bought this new microwave oven that trashes every AM radio in the house while it's running, So I built a Shielded loopstick antenna for my old RCA Radiola. It is untuned, Faraday shielded, and I used a mosfet amplifier with the drain capacitor coupled to the external ant input to the set. No impedance matching to speak of, but it works really well! No noise from the microwave and it picks up more stations than I care to listen to.
G. Gordon Liddy And don't forget Phil Hendrie. ;-) I've become hooked on Bill Bennet's "Morning in America". XM dropped the show 9/1 so I had do buy a subscription (via the internet) and an MP3 player with an FM modulator to get my daily fix. :-(
Yes, a separate short winding on the loopstick improves operation. The main winding is about 80 turns, and I added another winding of about 20 turns feeding the antenna input. I get a sharp peak on weak stations, and it picks up stations I can't get in the car, so it seems to be working better than the normal car radio setup.
There are 8 tunable coils inside the radio which is a Ford modle from
1980s. I tried tuning a couple coils near the antenna input, hoping to improve it further, but didn't have much effect.
I might try adding or subtracting a few turns from the secondary winding to see if I can improve it further.
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