Why is it that I have one of the portable receivers with a whip antenna that is about 2 feet long and can receive very well the short wave stations, but my ham receiver with a couple of feet of wire on the back receives almost nothing. When I hook the normal antenna to it, it receives vey well. The signal generator shows the sensitivity is very good. It is not just one ham receiver,but several.
My theory is that the portable receiver is matched for a high impedance short antenna, where the ham receiver is matched for a 50 ohm antenna.
Yes, that would account for it. The portable might connect the aerial to a tuned circuit so the termination is optimally matched for both resistance and reactance across the bands.
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~ Liz Tuddenham ~
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I know that, however my ham transceivers check out very well on most all frequencies.
However the portable picks up the hams with just the whip, where with wire the same length or even longer the ham transceivers picks up almost nothing. Checking them out on a service monitor, they are sensitive enough. However when the ham gear is used with a antenna only a few feet long tuned to frequency in a car they pick up lots of signals.
I suspect that your guess about impedance matching is probably "on the mark". Since the ham transceiver is normally hooked up to a 50-ohm (or so) trqansmission line, it's probably designed to terminate the incoming signal into 50 ohms or thereabouts when it's in receive mode.
If you hook a short wire to it, the short wire is going to have a very high (and very capacitive) impedance. The 50-ohm termination will look very much like a short-to-ground compared to that high impedance, and there will only be a trace of signal left for the receiver to detect.
A short-wave receiver is designed to work with a short antenna (wire or whip). The antenna is a lot shorter than a wavelength, and so there's no real need to terminate it into a matched impedance. The short-wave receiver can use a high-impedance receiving circuit (e.g. a JFET) and doesn't lose signal into an inappropriately-low termination impedance.
Some ham transceivers have a separate "receive antenna" jack - the Kenwood TS-2000 has one. These may be high-impedance inputs suitable for a wire or whip, or they might be 50-ohm antennas.
If you were to provide a small battery-powered buffer for your random wire (say, a J310 JFET set up as a source follower) you could reduce the effects of the mismatch and you'd probably find your transceiver able to receive quite nicely.
Your theory is absolutely correct :-) A good example is the AM band car radio. The great advantage of low impedance is that it is well suited to coaxial cables. In car radios there was an adjustable capacitor to compensate for the short length of the cable and front end tuning was good only on the station selected.
** But ( older) car radios had high impedance inputs on the AM band. A short run of coax is a pure capacitor at AM frequencies. The trim cap set the antenna coil to resonance while moving ferrite slugs did the actual tuning.
I don't recall the impedance of the car antenna coax, but it was special low capacitance/high impedance . Most common coax for radios and TVs are usually either close to 50 or 70 ohms and have about 2 to 3 times the capacitance per foot as the car coax.
Coax cable for AM car radios is AMC-62 modified for 125 ohms: The input to the AM receiver looks like a big inductor with an adjustable tuning capacitor in series. To the antenna, the tuning capacitor and coax capacitance to ground (9.5 pf/ft) form a voltage divider. The higher the coax capacitance, the less voltage will across the big inductor. Therefore, short coax cables and low capacitance coax cable are required. If one substituted common RG-58a/u coax cable (50 ohms), the capacitance would be 30 pf/ft resulting in a lower capacitor divider ratio (approx 1/3) or a -9.5 dB voltage loss.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
On Saturday, December 5, 2020 at 3:24:42 PM UTC-5, Ralph Mowery wrote: >
GM/Delco used RG62, which is 93 Ohms. I asked an EE from Delco about this about 45 years ago.
A lot of aftermarket car radio antennas in thee '70s used rg58, which lowered the sensitivity of the radio. Some had a series capacitor at the car radio end, to lower the capacitance across the input, but that became a voltage divider.
See my previous posting in this thread. The AMC-62 coax is 62 ohm RG-62/u modified for American Motors Corp by replacing the small center wire with an even smaller gauge wire. I couldn't find the gauge but I do recall taking apart a broken antenna and finding what looked like 40AWG. Broken center wires were probably common: However, that was tolerated because it improved the sensitivity of the AM radio.
Yep. I remember those, but at the time, I didn't understand the implications. For a time, I was working for a 2way radio shop installing disguise antennas in law enforcement vehicles. It was common to replace the AM/FM antenna with a "disguise" 1/4 wave stainless whip antenna and RG-58c/u coax cable. I soon discovered that the replacement AM/FM antennas were the same as the "disguise" antennas, except for the connector. When I started seeing Motorola car radio connector to PL-259 adapters, and replacement antenna kits shipped without connectors, it became obvious why they were using RG-59/u. Somewhat later, car antennas with built in diplexers became available. These had ports for AM, FM, VHF, and sometimes UHF radios. With the high power 2-way radios of the day, they tended to blow up AM/FM car radios, and sometimes burn out the diplexer. They are still sold today:
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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