'Hifi' Am antenna input impedance

[dr pepper]

I'm not listening to broadcast stations, they are an advert wasteground anyway, and fm is similar.
My eddystone picks up a lot more than the kenwood does, not sure what part of the kenwood makes am sound better maybe its the detector or something.
I'd expect a reasonable rx to be useable down to a micro volt, less than that is pretty good, 40uv is a little deaf, as you say am broadcast tends to be a strong signal anyway.
I have a tube set that has Afc on Am, never thought about how that works, maybe it has 2 detectors.

 

[unclejed613]

looking at other amateur rigs, as well as communications receivers, and it looks like making them somewhat deaf on the AM broadcast band compared to other bands is common practice. Icom's IC-R8500, which is one of the best receivers out there does it too (13uV on AM band, most of the rest of the bands are 2uV in AM mode, or 0.25uV in SSB on all bands, with the AM band sensitivity in SSB mode at 2uV). there are people that do AM DX listening, but it's not as commonly done these days. since AM stations are usually pretty strong, it kind of makes sense to "deafen" tuners on the AM band, because it's easy overload a sensitive front end, which causes a lot of other problems in a tuner. the amount of "deafness" designed into a receiver will often be a trade-off with other design goals, such as image rejection, intermodulation characteristics, etc... a tuner that can pull a 1 microvolt signal out of the air is no good if your local 50kW blowtorch station 100khz down the band bleeds through and maxes out the AGC so you can't hear anything else.

 

[Nigel Goodwin]

I'm not listening to broadcast stations, they are an advert wasteground anyway, and fm is similar.
My eddystone picks up a lot more than the kenwood does, not sure what part of the kenwood makes am sound better maybe its the detector or something.
I'd expect a reasonable rx to be useable down to a micro volt, less than that is pretty good, 40uv is a little deaf, as you say am broadcast tends to be a strong signal anyway.

You can't compare a HiFi receiver with a communications one, they are totally different things, with totally different purposes.

The Kenwood sounds 'better' because it's higher bandwidth, and hence less sensitivity - one reason communications receivers work better on poor signals is because they use narrower bandwidth, and as it's only intended for speech (and usually reduced bandwidth speech), that doesn't matter.

 

[dr pepper]

You probably right Nige.

Jed I had an issue with Dcf77 a while back, I wanted to pick the signal up & use it as a reference, but there is a station down the road 3khz higher that is a couple of 100kw and wipes it out.
My eddytone, alinco & poxy realistic all pick up both at the same time, but my sdr picks it up no problem even shows clear between them, shame its no use.

 

[unclejed613]

You can't compare a HiFi receiver with a communications one, they are totally different things, with totally different purposes.

true, but the fact that designers reduce the sensitivity within the AM band, even in communications receivers shows there's a trade-off in receiver performance between sensitivity and front end overload, and that's why so many receivers (even very good ones) are somewhat deaf in the AM band. it's even an issue with SDR radios, and many of them have band stop filters to attenuate both the AM and FM broadcast bands.

 

[dr pepper]

I wonder if some rxers use a reject filter for broadcast, to improve Imd elsewhere in the same band.
I chucked together an up convertor for my sdr, for listening to Vlf, it has a am broadcast filter.

 

[ronsimpson]

I wonder if some rxers use a reject filter for broadcast, to improve Imd elsewhere in the same band.

Back in my broadcast days; I built a high power FM transmitter in town. It was at the bottom end of the FM band. There was no channel 6 or 7 TV in town. We were outside of their viewing area. It turns out that many people were watching 6 & 7 with antennas. My strong signal was very close to TV-6 and my harmonic was at TV-7. Because 6&7 were out side our area I did not have to fix anything (FCC) but because I am a good neighbor I got the pleasure of adding a blocking filter on the back of every TV with in a mile. It was a simple LC that shorted out the antenna at my frequency. Connect the filter and adjust the L until 6 came in clear. I also added a low pass filter to my transmitter line that shorted out harmonics frequencies.

The hardest part of my job was listening to people tell me that the FCC (government) should not allow me to transmit. Then while I am fixing the problem many people would tell me how the city-government did not allow them to add a third bedroom and how the government should stay out of peoples lives. "This is the land of the free" and "how dare the government control people". and .......

 

[dr pepper]

Sounds like fun.
Here in the Uk somedays you can pick up radio 5 on Am at 9kc Vlf, I think thats a harmonic.

People tell me how I should do my job all the time, I've started telling them 'its a good job you know I dont', confuses them every time.