Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old December 4th 20, 04:00 PM posted to sci.electronics.repair
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Default Receiver sensitivity



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.


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Old December 4th 20, 04:48 PM posted to sci.electronics.repair
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Default Receiver sensitivity

Ralph Mowery wrote:

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.


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.

--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
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Old December 4th 20, 04:55 PM posted to sci.electronics.repair
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Default Receiver sensitivity

In article , 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.

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Old December 4th 20, 09:22 PM posted to sci.electronics.repair
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Default Receiver sensitivity

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.
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.
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Old December 4th 20, 10:46 PM posted to sci.electronics.repair
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Default Receiver sensitivity

In article t,
Ralph Mowery 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
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.






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Old December 5th 20, 12:31 AM posted to sci.electronics.repair
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Default Receiver sensitivity

On 04/12/2020 21:22, Clifford Heath wrote:
Your portable probably has a ferrite loopstick inĀ* it also.


Probably not for shortwave though.

--
Brian Gregory (in England).
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Old December 5th 20, 09:46 AM posted to sci.electronics.repair
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Default Receiver sensitivity

On 04/12/2020 17:00, Ralph Mowery wrote:


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.

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.
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Old December 5th 20, 08:12 PM posted to sci.electronics.repair
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Default Receiver sensitivity

bilou wrote:
===========


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.
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



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Old December 6th 20, 05:37 PM posted to sci.electronics.repair
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Default Receiver sensitivity

On Sat, 5 Dec 2020 15:24:32 -0500, Ralph Mowery
wrote:

In article ,
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
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:
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
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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