last night I dreamt of some bagels

[maradydd]

I ordered one of these the other day, and it showed up yesterday afternoon. Last night I went up to the roof of the physics building to test it out with the 20m dipole I built a few weeks ago. It works pretty well. I also figured out why my SWR meter apparently wasn't registering any activity -- the instructions for the meter specifically say to test your antenna with a constant CW (Morse) tone or the "calibrate" setting if your radio has one (which the one I was testing it with a few weeks ago did not). Unfortunately I don't have a keyer, so I tried sideband, and was perplexed when the needle on the meter didn't move.

(Brief digression for the non-radio people: SWR expands to "standing-wave ratio", and refers to the ratio of power that you're putting into your antenna over power reflected back at your radio from the antenna. 1:1 is ideal and pretty much impossible to achieve. Anything over 2:1 means you're losing significant signal strength, and at 4:1 or greater you run a serious risk of damaging your equipment. So, if you're building antennas, you want to tune them before you use them, which means getting an SWR meter, testing your antenna, making adjustments, and testing and adjusting until you're happy.)

At first I thought the meter was shot. I've used an SWR meter on 2m, 70cm and 6m before without the least bit of difficulty. Well, tonight I tried speaking into the mike while I tested the antenna, just for grins, and voila -- needle jumping all over the place. Which is how I figured it out: the meter instructions said to use CW because it needs a carrier signal. D'oh. FM without anybody saying anything = transmission that sounds like dead air but registers steadily on an SWR meter. SSB doesn't work because the amplitude will jump all over the place, and when nobody's saying anything, the amplitude will be zero (well, really, minimal due to whatever background noise the mike picks up).

(More digression: 20m, the wavelength for which I built the antenna, is one of the high-frequency [HF] bands, which range from 10m to 160m mostly by doubling but with a few odd inclusions like 12m, 15m, 17m and 60m. The HF bands are generally reserved for CW, packet, and single-sideband operation; FM is generally the province of the VHF and UHF bands. FM is suitable for working with an SWR meter because it transmits sound as variations in the frequency of a carrier wave; the amplitude stays constant. AM is less suitable, because while it has a constant-frequency carrier wave, the amplitude jumps around. SSB is wholly unsuitable for calibration because it has no carrier.)

This is all RF-theory stuff that I knew already in theory, but had never had cause to put into practice and therefore hadn't really integrated. I am such a goddamn engineer sometimes. So I think the Right Thing To Do is to either build a tone generator and test it that way, or just find a vacant piece of the band around 4am sometime and test the SWR using FM at the lowest output power I can get away with, because hey, the band allocation just says "20m phone", not "20m sideband". Nyaah. (FM has higher bandwidth than SSB, which is one reason why it isn't as widely used, especially on the lower frequencies. As an example, the band allocation for 20m phone is 14.225MHz-14.350MHz at my license class; for 2m phone it's 144.1-148.0MHz, which is a lot more room to play around in, so FM isn't quite as rude.)

Anyway, I managed to tune in a couple of people calling CQ on the CW bands (though unfortunately wasn't able to answer due to not having a keyer) and everything is cool now.

I also picked up one of these. In keeping with the butler theme, I have named it Edmund, as doissetep has informed me that Edmund Blackadder was a butler in the Regency season. I'm pretty happy with it, too.

Comments

[grepmaster.livejournal.com]

You've misunderstood my diagrams, there's no time component and no fourier transforms involved (I did not mention fourier at all) - they are simple plots of amplitude against radio frequency. On that kind of plot, it's exactly what AM does.

Well, ok. If one were to analyze the signal over a period of a microsecond, and plot that, then analyze it over the next microsecond, and change the graph, and so on, the result would be what you're talking about in terms of the changing AM frequency plot. However, measuring the frequency of the HF signal necessarily involves measuring it over a distinct time interval; if you just measure it at a single time point, then you don't have any oscillation, just a single amplitude. From there, it's purely arbitrary whether you decide to measure it over a microsecond and get the time-changing frequency plot, or measure it over a second and get the static transform. However, only the latter will tell you what actual frequencies will be broadcast off your antenna.

The plot you gave for FM is only valid over the longer time-interval; over the shorter time interval, the plot will be a single line at a frequency offset from the carrier by the amplitude of the modulating signal.

there is also a way to do something called single side band on AM

The term single sideband normally applies only to AM; SSB as applied to FM was AFAIK only used as a special speech scrambling technique during WWII. definition:

single sideband

(SSB) AM radio communication technique in which the transmitter suppresses one sideband and therefore transmits only a single sideband.

and:

Single-sideband modulation (SSB) is a refinement of the technique of amplitude modulation designed to be more efficient in its use of electrical power and bandwidth. It is closely related to vestigial sideband modulation (VSB) (see below).

Amplitude modulation typically produces a modulated output signal that has twice the bandwidth of the modulating signal, with a significant power component at the center carrier frequency. Single-sideband modulation improves this, at the cost of extra complexity.

...

The best way of thinking of SSB modulation is to first consider an amplitude-modulated signal. This will have two frequency-shifted copies of the modulating signal (the lower one is frequency-inverted) on either side of the remaining carrier wave. These are known as sidebands: either upper sideband (USB) or less commonly lower sideband (LSB).

To produce an SSB signal, a filter removes one of the sidebands. Most often, the carrier is reduced (suppressed) or removed entirely. Assuming both sidebands are symmetric, no information is lost in the process. What remains still contains the entire information content of the AM signal, using substantially less bandwidth and power, but cannot now be demodulated by a simple envelope detector.