Bill,
Thank you for the podcast and the loads of tribal knowledge from you and Pete! I recently started going through the SolderSmoke backlog, and I am just getting to the Colorburst Liberation Army episodes. I had some crystals for 40m laying around, so I started reading up and winding on a film canister.
Later, I thought I'd hop up the current episode of the podcast while I work, and I heard you guys talk about another push for the Michigan Mighty Mite. I happened to be salvaging bits from an recycling-pile VCR at the time, and I came across this crystal with a familiar label: 3.579545. The radio gods have spoken, so I will be winding a new coil for 80m.
I heard you mention the anonymous benefactor, but I have some aluminum foil, and I plan to brew the cap as well, there are a few options here: http://www.instructables.com/howto/variable+capacitor/
I thought I'd tell you a quick story anyway, because I just want to be a part of the fun. When I was little, our radios all had retractable antennas, and the TVs all had rabbit ears or loops. I would spend nights trying different materials and orientations to try and get better signal. At one point, I ran as much wire as I could in my brother's room (he had a big garage-sale cabinet tv), and we were getting channels from all over the place. We knew when each channel would be showing reruns of Star Trek, so we could catch an episode or two almost every day of the week. We even got some channels that I thought were only available on satellite. I had read in my box of Popular Mechanics magazines that some dish setups re-broadcast their signal on VHF/UHF frequencies with low power, I always suspected that's what we were picking up.
I didn't know much (or really anything) about the theory behind antennas at the time. I'm still a bit hazy, like with the 75 ohm / 50 ohm cable thing. I thought resistance was a function of length, how can different lengths of coax be the same ohm rating?
Anyway, I just wanted to let you know I'm new to homebrewing, and you guys are a big part of that!
73,
Dan, KC0ZIR
Northern Virginia (a little to the left of you, it sounds like)
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Excellent Dan!
Getting the 3.579545 MHz rock out of a dead VCR definitely adds mojo to the rig. Indeed, TRGHS! Thanks for sharing your Knack Story. I hope to meet up to you, perhaps at the hamfests of Northern Virginia. In my capacity as Grand Poohbah and Arbiter of Capacitor Eligibility, I hereby deem you ELIGIBLE for a variable capacitor. Our secret benefactor will be contacting you.
73 Bill
Posts
Hoping Dan reads this. Coax impedance is set by the ratio of the diameter of the center conductor to the diameter of the shield. It is the resistance you would measure if you had an infinite length of the cable and measured with an AC signal. Most folks just take the manufacturer's word for it. For most ham purposes 50 and 70 ohm cable are interchangeable. This means you can get some bargains on cable intended for CATV.
ReplyDeleteChip v.
Chip,
ReplyDeleteI just noticed this ended up on the blog, thanks for the reply! I've been reading about impedance [1] while studying for my general. I didn't realize that impedance was the hypotenuse of a right triangle with resistance and reactance legs.
The way I'm thinking about impedance matching now, it makes sense to me that the impedance of the antenna and of the radio should match. If I have a simple circuit with a voltage source and two resistors in parallel, and the resistors are of the same value, the voltage drop and current should be the same across both of the resistors. With impedance, if the impedance is mismatched, the power will not be equal between the radio circuit and the antenna.
-Dan, KC0ZIR
[1] http://www.phy.davidson.edu/instrumentation/Files/NEETS/Mod02%20-%20AC%20and%20Transformers.pdf