'Hifi' Am antenna input impedance

[dr pepper]

Whats the typical input impedance for a stereo systems Am aerial?
Fm states 75r but Am doesnt.
The schem for the receiver shows a fet buffer for the Am with a 1 meg r input imp, just before that though theres a impedance matcning transformer, it depicts less turns on the primary so the input imp will be less that the 1 meg r.
I guess I could meter the inductance of the primary / secondary and calc from there.
I want to connect a miniwhip to a 'hifi' seperates Am receiver.

 

[rjenkinsgb]

If it's a normal coaxial connector, it's likely to be a 75 Ohm antenna.
Impedance matching / SWR is not critical on receive antennas.

In practice, just about any long wire will work regardless of impedance or tuning.

 

[ronsimpson]

If there is a receiver and antenna (with no coax in between) then the impedance is not critical.
If there is a coax or transmission line in between then impedance has effects.
If the coax is less than 1/8 wavelength the effects will be small. AM has a long wavelength while FM is much shorter.
A filter can be a piece of coax cut at 1/2 wavelength. Depending on how bad the mismatch and how many wavelength you can have a band block filter that you don't know is there.

 

[Nigel Goodwin]

There's no impedance matching on am, it's designed for a simple 'long wire', who's impedance could be anything.

If anything, it's a highish impedance input, to avoid loading the crude aerial.

However, quality on am is so abysmal, why bother? - I can't remember the last time I ever used am, certainly not this century, and not near the end of the last one either.

 

[dr pepper]

Coax transformer, yes I remember those.
Ok then I'll assume its a high impedance, I expect that on a wire ant thats much shorter than the wavelength.
Wasnt sure if the Am input was sposed to be a tuned loop operating into a load resistance or something.

 

[ronsimpson]

Back when I built radio stations:
I was working on a transmitter one night and wanted to listen to music. I connected my FM radio to the transmitting coax & antenna. Coax was 3 inch copper pipe that ran 570 feet up to a resonant antenna with a gain of 10. Almost every frequency has a signal from some far away station. A $100,000 antenna should work better than a coat hanger.

 

[dr pepper]

I used to maintain a Lw transmitter, but never touched an antenna.

 

[languer]

An AM radio receiver will likely use a FET input with high impedance. The antenna is normally some form of tuned loop (magnetic in most cases). AM radio is really about ground waves so a long wire should work because the receiver should be high impedance and to get low impedance on a wire antenna would require such a long wire that it would be very impractical (thus the use of tuned loops).

https://www.ccrane.com/expert-advice/all/1/article/18

 

[unclejed613]

there's no standard input impedance for the AM part of the tuner. as people have already mentioned, a long piece of wire is what is usually used. some more modern receivers come with a small (5 or 6 inch diameter) loop antenna. the advantage of the loop antenna is that you can null out local interference sources by turning the loop until it's broadside to the source of noise. the 1 Meg resistor on the gate of the input FET allows the FET to be self biased.

 

[Nigel Goodwin]

An AM radio receiver will likely use a FET input with high impedance. The antenna is normally some form of tuned loop (magnetic in most cases). AM radio is really about ground waves so a long wire should work because the receiver should be high impedance and to get low impedance on a wire antenna would require such a long wire that it would be very impractical (thus the use of tuned loops).

Sorry, but pretty well everything in your post is incorrect.

It would be very rare for an FET to be used, I don't recall coming across any sets which did?.

Occasional sets did use an external loop aerial, and these (obviously) fastened to a pair of screw connectors on the back of the unit. However, they weren't much use as you're effectively placing the aerial where signals are weakest, and interference is greatest. The vast majority simply gave you a single screw connection for a long wire, and some had a ferrite rod mounted on the back, and while it's better than a crude loop it still suffers from it's poor siting.

But I suspect that hardly anyone ever used the AM facilities, due to their pathetic quality.

 

[languer]

It would be very rare for an FET to be used, I don't recall coming across any sets which did?.

I respect your point of view, but it is just that; your point of view. AM radios I worked (mostly for failure analysis) for cars used almost exclusively FET inputs, a Motorola chipset (or Philips), and a ferrite loop antenna (not an aerial). On two different ADF system (the AM radio for aircraft) split between FET input (JFET) for the older system, and bipolar based-mmic for the newer system. But I am sure there are many ways to skin the same cat. I just remember it because that was my first (and likely last) exposure to JFETS and the Motorola chipsets.

 

[Nigel Goodwin]

It's my 'point of view' based on over 46 years of repairing radios, TV and HiFi equipment, not so much car radios though - apart from particularly old ones.

It's possible that later car radios used FET's, due to the severe limitations of their aerial systems for AM.

 

[dr pepper]

The receiver has a very ordinary looking loopstick, referred to on the schem as a matching transformer.
The Am ant in goes to 3 turns around this and then to ground.
The Am ant does actually go to a fet stage., after a varactor which I assume is a tracking pre selector.

 

[Sinedup]

Size counts - the longer the better.
While working in Durban (coastal city) during the 80's, I bought one of those all-band "world receivers" which covered BCB FM, SW1-6, MW AM, and LW.
The packaged AM antenna was a simple length of PVC-insulated copper wire with a 'pin' plug at receiver. The booklet advised that "for better reception any longer length of wire may be added, orientated for best reception".
Mine could pick up the entire globe, along with all the pops, squeals, phasing etc.
VOA and BBC (to RSA) were generally excellent reception.
ADD: As for impedance, I remember a (PE?) magazine had an AM active impedance matcher (never got to building it) which mentioned matching any 'hunk' of metal railing, downpipe etc to the "high input impedance" of an AM monopole input, planet earth being the 'virtual ground'.

 

[unclejed613]

for the AM BCB, most wires are going to be "very short" compared to a dipole antenna (468 feet for 1Mhz), so the input circuit on most AM receivers is primarily sensing voltage at the antenna terminals.

 

[dr pepper]

Thats why this one has a fet on the input then.
This rx despite being 'hifi' is actually a load of crud, sensitivity and selectivity are pants, I'm going to find a better one.
I have a old fisher that not only doesnt have a ferrite loopstick its also in a metal box so noise on my bench isnt going to destroy reception likei t does with this heap.
The front of the receiver says kenwood, and the audio community seems to liek it, but the interior resembles a badly made 70s bargain basement set.

 

[unclejed613]

i'm curious... what model is the Kenwood?
AM tuners, even in high end receivers, don't have a lot of sensitivity in the AM band. they're designed more to keep the front end from overloading than they are to receive weak signals. a lot of AM stations in the USA have added digital HD subcarriers to their transmitters, so there's a LOT of noise in the AM band that wasn't there before.

 

[dr pepper]

Its actually a trio kt1300b, however google has it as a kenwood kt1300b, they are one and the same.
Electrotanya has a schem for it, 7th or 8th reply on google search.
It does have some agc, it uses a chip which is Fm If, and Am osc / If / detect.
Digital here in the Uk is not on Am or Fm broadcast, we have dab which has its own band completely, somewheres around 220Mc's.
Also Am channels here have 9kc split, not 10, not a problem for the kenwoods twist & tune, I'd say its a better Am rx than an Fm.
Note the am receiver I have with a fet front end is a fisher Fm860.

 

[Nigel Goodwin]

Its actually a trio kt1300b, however google has it as a kenwood kt1300.

Trio was the name used in the UK by Kenwood, as the Kenwood name was already registered, by Kenwood Food Mixers.

In later years the Kenwoods came to an arrangement (the details of which were never mentioned), and the Trio name disappeared, and they started using Kenwood as well. Funnily enough I bought a Trio 3 head cassette deck, then a few months later I bought the matching amplifier (which I still have), and in the interim the name change had occurred.

As has been mentioned, AM is crap in HiFi systems, I've no idea why they ever bothered with it.

If FM is poor, then there's probably a fault - it's usually pretty good, although I think the KT1300 was a very budget and low spec model?.

 

[unclejed613]

the AM sensitivity is 20uV, (signal level required for a 10dB s/n ratio) which is really bad. just to keep in "in the family", i'll compare it to my Kenwood TS-680 amateur transceiver. in the AM band only, on the TS-680, the sensitivity is 39uV in AM mode. in SSB mode, however it's 4uV. outside the AM band (below 500khz and above 1600khz) the sensitivity is 2.5uV in AM mode, and 0.25uV in SSB mode., so even in a very good communications receiver, the tuner is intentionally desensitized within the AM broadcast band, and that's also true for the receiver in SSB mode, but in SSB mode the sensitivity is much better than 20uV. since i can do a pretty good job of zero-beating my radio, i use SSB mode for listening to AM signals.