Re: Tube Hi-Fi directional AM antenna?


Nevertheless... they WORK, to quickly analyze a front end like a
barn door that`s trying to de-mod a crashing signal!

Have you checked what is inside a coaxial attenuator ?

Don`t scoff without testing.. not everyone has a service bay in
their basement and university pass in radio.. they have to use
what`s to hand... :) lol.



--- In, "gabevee" <gabevee@...> wrote:

--- In, "Lee" <leo1949uk@> wrote:


Also, kilohm to megohm resistances are absurd for FM, as the impedance for the FM antenna is 300 ohms! Even for AM megohm values are absurd.
I have some belling-lee coaxial attenuators that plug into the t/v
socket and the coax plugs into the attenuator.. the smallest being
1 kilohm... the highest attenuator being 1 megohm .. the antenna
coax is 75 ohm !!!! Antiference is the manufacturer !!!

No, my suggestion of using the baluns to match the receiver's 300 ohm FM to the 75 ohm coax to use the attenuator I suggest is the correct one, considering the user said he didn't want to make one.
European equipment generally 75 ohm ccoaxial cable the antenna
already has a balun fitted... different to 300 ohm balanced in the
USA equipment has both., I repeat, those values are absurd for attenuators. I suspect you are seeing the frequency range as resistance values. The numbers you specify are impossible for 2-15 dB attenuation. They are likely 5 kilohertz to 1000 megahertz.

Attenuation for RF like AC is virtually the same as for DC. 1 kilohertz in line with 75 ohms makes the signal virtually zero. And we're talking about microvolt levels.


The internal circuit is made of resistors as you seem to know. However the 75 ohm impedance must be matched. That means the input and output resistor must be 75 ohms. The resistor from the input to the output, to drop 6 decibels, which his half the voltage, must be 75 ohms! It is a simple voltage divider circuit.

Z2/Z1+Z2 X Vin

The input 75 ohms doesn't count unless you worry about the current, which is in the pico range. Inputs to RF in receivers are generally only concerned about voltage. So the input resist provides a fixed load for the signal to prevent standing waves. So we have 75 ohms plus 75 ohms for 150 ohms. 75 ohms over 150 ohms times the input voltage is the same as 1/2 the input voltage. 6dB drop. Voltage. The dB level is different for power (femtowatt).

1 kilo to 1 megohm is *ABSURD*.


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