Acceptable registrations in the queue through March 16 at 11:00a ET have now been activated. Enjoy! -M.W.

Terms of Use have been amended effective October 6, 2019. Make sure you are aware of the new rules! Please visit this thread for details: https://www.mibuzzboard.com/phpBB3/view ... 16&t=48619

AM reception question

The technical side of broadcasting. Think IBOC is a sham? Talk about it here! How about HDTV? Post DX reports here as well.
User avatar
Ben Zonia
Posts: 2143
Joined: Sat Jun 03, 2006 10:35 pm
Location: Honor

Re: AM reception question

Post by Ben Zonia » Sun Aug 23, 2020 11:10 am

So, the incandescent bulb acts like a dummy load and doesn't radiate? I remember our neighborhood amateur using that for his Heathkit transmitter that put out about 90 watts CW, designed as a "starter" transmitter for Novices. When his call changed from WN8*** to WA8***, he built a Knight Kit VFO and linear. I think he also built a better dummy load then.

It may have been this one.

Image


"I had a job for a while as an announcer at WWV but I finally quit, because I couldn't stand the hours."

-Author Unknown

User avatar
SolarMax
Posts: 702
Joined: Sat Dec 04, 2004 5:59 pm
Location: 313

Re: AM reception question

Post by SolarMax » Mon Aug 24, 2020 11:48 pm

We know that a light bulb does radiate, just a little bit. It's just that, to not generate QRM on the bands (especially if our shacks were in the basement), many of us used one in place of a real dummy load to check out our transmitters. Those tube finals and pi network output stages could handle the wildly varying impedance that a bulb's filament would present at different RF levels.

Unfortunately we've just missed the annual "Light Bulb QSO Party" this past weekend (8/22-23), the purpose of which is to build and use an antenna constructed with a lightbulb as a key component ... and to promote understanding and practical application of antenna matching concepts that allow a lightbulb to be used as a radiator in two way radio communications.
Info on that event and how to start getting ready for the 2021 event at https://hamsignal.com/blog/lightbulb-qso-party-2020



User avatar
Ben Zonia
Posts: 2143
Joined: Sat Jun 03, 2006 10:35 pm
Location: Honor

Re: AM reception question

Post by Ben Zonia » Tue Aug 25, 2020 12:10 am

That Knight Kit should also read Heathkit. I remember him showing me the variable capacitor in the VFO. It had a physical "governor" that prevented the VFO from operating out of band. I guess a suitable lesser capacitance variable capacitor wasn't available.


"I had a job for a while as an announcer at WWV but I finally quit, because I couldn't stand the hours."

-Author Unknown

User avatar
SolarMax
Posts: 702
Joined: Sat Dec 04, 2004 5:59 pm
Location: 313

Re: AM reception question

Post by SolarMax » Tue Aug 25, 2020 4:08 pm

Ben Zonia wrote:
Tue Aug 25, 2020 12:10 am
...the variable capacitor in the VFO....had a physical "governor" that prevented the VFO from operating out of band. I guess a suitable lesser capacitance variable capacitor wasn't available.
I'm guessing the variable cap's physical range was mechanically limited so that only the most linear part of its travel was used to tune across a band. Otherwise, the dial calibration at one end of the band or the other might be "squished." Some old, cheaper AM broadcast radio dials often showed the upper AM band frequencies all packed together and lower ones spread apart. A trimmer capacitor could be incorporated to do some "stretching out" and linearize the calibration. A big selling point of Collins and some other high-end ham and commercial gear was a PTO (Permeability Tuned Oscillator), which used a geared drive to push or pull a ferrite or iron core in and out of a coil form, a system which provided nearly perfect linearity. Heathkit later had an "LMO" (Linear Master Oscillator), which achieved a similar result.



User avatar
Ben Zonia
Posts: 2143
Joined: Sat Jun 03, 2006 10:35 pm
Location: Honor

Re: AM reception question

Post by Ben Zonia » Tue Aug 25, 2020 5:16 pm

The first FM radio we ever had, in a Magnavox Stereo Theater, used a ferrite core, but it was changed by a pulley system ganged with the dial cord which went around the flywheel to adjust the AM Variable Capacitors. I think the ferrite core was weighted and that weighting made the core go down, while the pulley lifted it up. The reason I know so much about it is it broke from too much DXing, and I had to fix the broken and frayed dial cord numerous times. The family wasn't amused. OTOH, I think the rotary detent VHF tuners, and most of the controls, benefitted from heavier use, as long as it wasn't too fast. It cleans the oxides off the contacts with use.


"I had a job for a while as an announcer at WWV but I finally quit, because I couldn't stand the hours."

-Author Unknown

k8jd
Posts: 603
Joined: Fri Jun 28, 2013 3:35 pm
Location: Commerce, MI

Re: AM reception question

Post by k8jd » Wed Aug 26, 2020 12:27 pm

Re; Lightbulb RF Load.
YES they radiate a bit,
I had a Heathkit Cantenna dummy load, fed with coax when testing . Fairly well shielded !
I was playing with a home made audio compression amp and listening to myself with headfones and heard some one calling me in the background...seems like a friend who lived a mile from me heard my tests on ten Meters and we actually conducted A QSO, Dummy Load to Dummy load ! !



k8jd
Posts: 603
Joined: Fri Jun 28, 2013 3:35 pm
Location: Commerce, MI

Re: AM reception question

Post by k8jd » Wed Aug 26, 2020 12:32 pm

SolarMax wrote:
Tue Aug 25, 2020 4:08 pm
Ben Zonia wrote:
Tue Aug 25, 2020 12:10 am
...the variable capacitor in the VFO....had a physical "governor" that prevented the VFO from operating out of band. I guess a suitable lesser capacitance variable capacitor wasn't available.
I'm guessing the variable cap's physical range was mechanically limited so that only the most linear part of its travel was used to tune across a band. Otherwise, the dial calibration at one end of the band or the other might be "squished." Some old, cheaper AM broadcast radio dials often showed the upper AM band frequencies all packed together and lower ones spread apart. A trimmer capacitor could be incorporated to do some "stretching out" and linearize the calibration. A big selling point of Collins and some other high-end ham and commercial gear was a PTO (Permeability Tuned Oscillator), which used a geared drive to push or pull a ferrite or iron core in and out of a coil form, a system which provided nearly perfect linearity. Heathkit later had an "LMO" (Linear Master Oscillator), which achieved a similar result.
The "linear dial calibration " either had specially designed variavle cap plate shaping or used a tuning system that moved slugs in and out of coils.



k8jd
Posts: 603
Joined: Fri Jun 28, 2013 3:35 pm
Location: Commerce, MI

Re: AM reception question

Post by k8jd » Wed Aug 26, 2020 12:38 pm

SolarMax wrote:
Mon Aug 24, 2020 11:48 pm
We know that a light bulb does radiate,
Unfortunately we've just missed the annual "Light Bulb QSO Party" this past weekend (8/22-23), the purpose of which is to build and use an antenna constructed with a lightbulb as a key component ... and to promote understanding and practical application of antenna matching concepts that allow a lightbulb to be used as a radiator in two way radio communications.
Info on that event and how to start getting ready for the 2021 event at https://hamsignal.com/blog/lightbulb-qso-party-2020
I think many of the lightbulb antennas used consist of a Quarter wave of wire attached between the antenna matching tuner and the light.
Back in the '60s, I used to run an end fed wire right out of the tube transmitter and wrapping a few turns of the wire around a burned out fluorescent lamp tube did light the room in Morse code blinks ! ;D



Post Reply Previous topicNext topic