Q and crystal radios

[dr pepper]

As I said in my post #16 if you put a resistor across the L/C you lower the Q, in this case the load is the resistor.
As Col put it the higher voltage as soon as you load it will vanish.
Which is why I mentioned a tuned 1/4 wave whip, such a antena only generates uVolts, however its much 'stiffer'.

 

[spec]

Quarter wave length antennas are used for very high radio frequencies (VHF) and above but there are practical implications with using tuned antennas at low (LW), medium (MW), and short wave (SW) radio frequencies. For example, take the 1,500 meter band, a quarter wave length antenna would be 375 meters long.

An effective way around this is to use ferrite rod antennas as found in portable long, medium and short wave radios.

spec

 

[Colin]

"if you put a resistor across the L/C you lower the Q,"
Let's explain that more simply.
I said you are only pushing Big Momma will tiny little pushes and the energy from the radio station is very small. If you put a resistor across the tuned circuit, you remove more energy than can be delivered.
What happens?
The very high voltage produced by the "little pushes" is reduced and since the ratio of the microvolts to the 2v produced is given the value "Q" (quality factor), the end result is a reduction in the value of Q.

"Which is why I mentioned a tuned 1/4 wave whip, such a antena only generates uVolts, however its much 'stiffer'."

Talking about antennas.

I went to fix a Strongberg Carlson TV 40 years ago.
"Where's the antenna" I said.
"It's sticking in the antenna hole."
They had a wire coat-hanger sticking in the antenna point at the back of the TV.
"That won't work" I said.
"Yes, it does"
I fixed the TV and the picture was absolutely perfect.
Only one wire coat-hanger. No balanced antenna. No full-length antenna "elements." No proper antenna engineering. Just an old mangled wire coat-hanger.

 

[dr pepper]

Some cars here in the Uk were made with a coathanger as the aerial.

 

[spec]

Some cars here in the Uk were made with a coathanger as the aerial.

Yeah, coat hangers have been make-shift antennas for years. When we first got married we had a 14 inch black-and-white 1955 gold-fish-bowel Bush TV with beautiful wood veneer cabinet and coat hanger antenna. If an automobile went by, on the road outside the picture would turn into snow, as it would if you were in certain parts of the room.

If the misus wanted to watch a particular program the set had to be switched on half an hour before viewing started so that it was at peak of its performance when the program started and I had to spend a ages adjusting the coat hanger for maximum signal.

As an aside, when we got another TV, I stripped the Bush for components. All of the carbon resistors were 70% to 100% high and the capacitors were all very low and often leaky. The 100uF reservoir capacitors were as leaky as hell and measured about 10uF when cold. How the set ever worked I will never know, especially as most of the valves (tubes) were way below their spec on conductivity too.

When the Bush was replaced, programs were no where near as interesting.

I wish I had the Bush now.

spec

 

[Tony Stewart]

In the early 60's we had moved to the far north in central Canada for an 18 mos period when my Dad was Staff Seargent in the RCMP. While in Grade 2 at the time , we had no TV broadcasters except for 500 miles to south in Winnipeg,,but due to the transfer we had a TV. I figured out out to tuned the UHF TV to pickup the harmonic of the police radio with rabbit ears and on hot summer days,,I could pick up Texas TV from many ground to cloud skips.

THen 15 yrs later armed with my little EE degree and years of RF experience, when a bunch of guys were over to watch the Superbowl and cable TV stopped in the last quarter and being in the basement the tuned coat hanger was nyet quite enough, so I would touch the coat hanger with one finger and tune the position of one leg and arm to get enough signal to watch the game until cable signal was restored.

My first 1/2 wavelength antenna design circa '76, was on a Black Brandt sounding rocket which spun at a couple hundred RPM so 3/8" braid roll,pressed flat was tuned in the lab with stretched wax harness string to cut within 1mm , the length for the VHF channel with >20 dB return loss for good matching. Then it was coiled up in a spool and when the nose cone was ejected, the antenna would stretch out to provide the data telemetry signal from a few hundred miles up over Hudson Bay, with a perfect signal.