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Receiver sensitivity
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Ralph Mowery
Dec 4, 2020, 11:00:16 AM12/4/20
to
While not really a repair,but more of a question.
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.
Liz Tuddenham
Dec 4, 2020, 11:50:12 AM12/4/20
to
to a tuned circuit so the termination is optimally matched for both
resistance and reactance across the bands.
--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
Ralph Mowery
Dec 4, 2020, 11:55:43 AM12/4/20
to
In article <ljnksf9k53i46g4i1...@4ax.com>, ken1943
@invalid.net says...
>
> Amateur Radio equipment is built to favor ham frequencies and not
> broad banded like your general equipment.
>
>
>
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.
@invalid.net says...
>
> Amateur Radio equipment is built to favor ham frequencies and not
> broad banded like your general equipment.
>
>
>
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.
Clifford Heath
Dec 4, 2020, 4:22:20 PM12/4/20
to
On 5/12/20 3:00 am, Ralph Mowery wrote:
> 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.
> 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.
> 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.
Your portable probably has a ferrite loopstick in it also.
> short antenna, where the ham receiver is matched for a 50 ohm antenna.
Dave Platt
Dec 4, 2020, 6:08:07 PM12/4/20
to
In article <MPG.3a3434126...@news.east.earthlink.net>,
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.
Ralph Mowery <rmower...@earthlink.net> wrote:
>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
>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.
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.
Brian Gregory
Dec 4, 2020, 7:31:29 PM12/4/20
to
On 04/12/2020 21:22, Clifford Heath wrote:
> Your portable probably has a ferrite loopstick in it also.
Probably not for shortwave though.
> Your portable probably has a ferrite loopstick in it also.
--
Brian Gregory (in England).
bilou
Dec 5, 2020, 4:46:11 AM12/5/20
to
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.
Phil Allison
Dec 5, 2020, 3:12:31 PM12/5/20
to
bilou wrote:
===========
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.
..... Phil
===========
>
>
> > 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.
> >
> 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.
>
> > 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.
> >
> 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.
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.
..... Phil
Ralph Mowery
Dec 5, 2020, 3:24:42 PM12/5/20
to
In article <e40e544b-0dec-41a6...@googlegroups.com>,
palli...@gmail.com says...
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.
palli...@gmail.com says...
>
> ** 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
> ** 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.
>
> ....
>
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.
Jeff Liebermann
Dec 6, 2020, 12:37:54 PM12/6/20
to
<https://www.commscope.com/product-type/cables/coaxial-cables/automotive-cables/item469776350/>
<http://objects.eanixter.com/PD357800.PDF>
<http://www.tian-jie.com/CABLES_Automobile-Antenna-Cable-Series_255_266_list.htm>
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.
--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Michael Terrell
Dec 16, 2020, 2:30:42 AM12/16/20
to
On Saturday, December 5, 2020 at 3:24:42 PM UTC-5, Ralph Mowery wrote:
>
> 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.
GM/Delco used RG62, which is 93 Ohms. I asked an EE from Delco about this about 45 years ago.
>
> 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.
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.
Jeff Liebermann
Dec 16, 2020, 1:26:08 PM12/16/20
to
On Tue, 15 Dec 2020 23:30:39 -0800 (PST), Michael Terrell
<terrell....@gmail.com> wrote:
>On Saturday, December 5, 2020 at 3:24:42 PM UTC-5, Ralph Mowery wrote:
> >
>> 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.
>GM/Delco used RG62, which is 93 Ohms. I asked an EE from Delco about
>this about 45 years ago.
See my previous posting in this thread.
<terrell....@gmail.com> wrote:
>On Saturday, December 5, 2020 at 3:24:42 PM UTC-5, Ralph Mowery wrote:
> >
>> 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.
>GM/Delco used RG62, which is 93 Ohms. I asked an EE from Delco about
>this about 45 years ago.
<https://groups.google.com/g/sci.electronics.repair/c/PNT_pCc-a3A/m/XEJLOtNnCAAJ>
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:
<https://www.jeepforum.com/forum/f176/radio-antenna-went-bad-how-replace-184449/>
However, that was tolerated because it improved the sensitivity of the
AM radio.
>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.
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:
<http://uscomm.atwebpages.com/Embedded%20Antenna%20systems.htm>
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