When researching the cost of high impedance coax as a feedline, it
apears globs of coax induced capacitance seriously degrates the
ability to tune the circuit. Under these conditions, resonating
frequencies requires extremely low inductance values.

How do people get around this problem in a receive only situation?

thanks,

mike

If your antenna is very short in terms of wavelength, then shunt
capacitance will reduce the amount of received signal. Solutions include
running the antenna wire directly to the receiver, using a preamp at the
antenna, or using a short length of special high-impedance coax as
feedline. The latter is what's commonly done with automotive AM radios.

If the antenna isn't very short, then "tuning" is done by means of a
matching network, which can compensate for mismatched coax line and the
impedance transformation it causes.

Roy Lewallen, W7EL

mike wrote:
When researching the cost of high impedance coax as a feedline, it
apears globs of coax induced capacitance seriously degrates the
ability to tune the circuit. Under these conditions, resonating
frequencies requires extremely low inductance values.

How do people get around this problem in a receive only situation?

thanks,

mike

mike wrote:

When researching the cost of high impedance coax as a feedline, it
apears globs of coax induced capacitance seriously degrates the
ability to tune the circuit. Under these conditions, resonating
frequencies requires extremely low inductance values.

How do people get around this problem in a receive only situation?

thanks,

mike

Another solution is a step down Z converter at the short antenna's
base. Typically a gate input FET stage- provides a bit of gain, but more
importantly, lowers the impedance. Then the issue of IM must be dealt
with if it is a heavy RF environment.

Dale W4OP

Mike,

You seem to be talking about situations like old time AM car radios, where
the feedline was really used as a shielded wire, and not a transmission
line. They got away with it, because the feedline was very short, and added
less than 100 PF of capacity.

When used as a transmission line, the line is terminated in, or at least the
same order of magnitude impedance as the the coax. The coax has both
capacitance, and inductance. So, if you connect a 50 Ohm antenna to 100 feet
of RG58 coax, the impedance you see at the other end is 50 Ohms, and you
don't have to worry about the fact that there is also 2800 PF of
capacitance. A receiver input generally has a transformer, or other device
that transforms the 50 Ohms to hundreds, or a few thousand Ohms. In
connecting a transmission line to a parallel tuned LC circuit, you don't
connect the line to the top of the LC. Rather you connect it to a tap near
the bottom of the inductor, or you add a second winding to the inductor to
make it into a transformer.

Tam/WB2TT

"mike" wrote in message
...
When researching the cost of high impedance coax as a feedline, it
apears globs of coax induced capacitance seriously degrates the
ability to tune the circuit. Under these conditions, resonating
frequencies requires extremely low inductance values.

How do people get around this problem in a receive only situation?

thanks,

mike

On Sun, 21 Sep 2003 23:45:09 -0400, "Tarmo Tammaru"
wrote:

Mike,

You seem to be talking about situations like old time AM car radios, where
the feedline was really used as a shielded wire, and not a transmission
line. They got away with it, because the feedline was very short, and added
less than 100 PF of capacity.

When used as a transmission line, the line is terminated in, or at least the
same order of magnitude impedance as the the coax. The coax has both
capacitance, and inductance. So, if you connect a 50 Ohm antenna to 100 feet
of RG58 coax, the impedance you see at the other end is 50 Ohms, and you
don't have to worry about the fact that there is also 2800 PF of
capacitance. A receiver input generally has a transformer, or other device
that transforms the 50 Ohms to hundreds, or a few thousand Ohms. In
connecting a transmission line to a parallel tuned LC circuit, you don't
connect the line to the top of the LC. Rather you connect it to a tap near
the bottom of the inductor, or you add a second winding to the inductor to
make it into a transformer.

Tam/WB2TT

FYI - I am a newbie SWL

OK, that answers my question. I now see for a transmission line the
coax also has inductance value which balanance out the extra
capacitance added by the 50 foot run of RG58 itself.

I was only looking at the capacitance value and it was driving me
crazy to understand why this wouldnt make tuning the circuit nearly
impossibe. I thought I was dealing with thousands of pf.

I am using a homebrew PI network tuner at the receiver end of my Sony
portable.

To get rid of household noise I want to move from an outside random
wire fed directly to the tuner to a coax line feeding feeding the wire
into the house , then to the tuner.

The schematic of my tuner looks like this:

http://www.qsl.net/dl2lux/fish/fishpi_e.html

Eingang = entrance, Ausgang =exit, Masse = ground

After translating those German words I just realized my transmission
line differs as it comes into the tuner and connects to the right side
capacitor movable vanes which is connected to the inductor tap switch.
So mine seems backwards in regards to the schematic. how much
difference would this make?

Mike

On Mon, 22 Sep 2003 14:40:24 GMT, mike wrote:

So mine seems backwards in regards to the schematic. how much
difference would this make?

Mike

Hi Mike,

Machts nichts.

73's
Richard Clark, KB7QHC

Why does everyone always worry about the parallel capacitance of a TL? Why
don't they worry about the series inductance? DUH!

73, Jack, K9CUN

Ok, I guess I have a handle on things now.
I am a newbie SWL still in learning mode.

To properly match the impedance of coax I must use a matching
transformer (sometimes 'incorrectly' called a longwire balun).

Guess I will have to take a look at the ICE 180, MLB, UMB...etc.

So with the tranformer my PI network tuner should resonate better (?)

I plan to keep using it regardless given its recently added back to
back voltage limiting diodes between input and ground and a static
bleed off resistor for DC to ground.

This is what I have going now and it has reduced noise.

I am running a 12 guage wire from the ground banana plug of my antenna
tuner to a copper wire brade that connects my TV antenna to a ground
rod. This composes one end of my coax ground to eliminate common mode
currents.


The other end is where the coax meets my 13 meter random wire hung on
a wooden fence.

Lacking a proper ground rod, I soldered a short length of 12 guage
wire from the coax sheild to the bottom of a diced up coat hanger (16
inches in length) and stripped away the 12 guage wire sheild for the
length of the hanger segment (exposing the copper). This I plunged
into the resonably moist soil under a lylac tree.

Hodge podge for sure. But I'm on a budget till my new job starts next
week.

Thanks for all the help,

Mike

Mike,

You definitely sound like me about 10 years ago. I started with a RS DX-390
radio and went from there. You didn't mention what kind of Sony portable
you had, but the first thing I realized (regardless of antenna) -- was to
get a better SW radio. You did say you're on a budget and that makes it
frustrating at times. But if you're careful, you can find a good deal on
ebay or other sites. I've seen Drake R-8 radios sell for around $250.
(They sold for $1200 in late '93.)

As for a 'good' SWL antenna... I picked up two of the RS shortwave antenna
kits (@ $10 ea) and bought a Palomar balun with RG-8 coax. I used to have a
Drake R-8 but traded it (stupidly) for something else. I've had a Yaesu
FRG-100 but now own an Icom R-75.

I can recieve pretty well, but I too am bothered by noise. I believe mine
to be more atmospheric rather than homemade QRM. I found that antenna
height is pretty important. My 100ft longwire antenna sits anywhere from
20-35ft above ground -- never going below 20ft.

You didn't mention what kind of programs you are listening to? If you're
listening to SW broadcasts on AM, then you'll hear much more 'noise' on AM
then you would on sideband. If you're trying to listen to military or
other HF sideband traffic, then you might be disappointed. I could barely
get ham stations on my DX-390! When I received my Drake R-8, I was amazed
at how much traffic was out there on HF -- that I couldn't hear before. The
quality of radio make a huge difference. ;o)


-Nick
n7zya

"mike" wrote in message
...
Ok, I guess I have a handle on things now.
I am a newbie SWL still in learning mode.

To properly match the impedance of coax I must use a matching
transformer (sometimes 'incorrectly' called a longwire balun).

Guess I will have to take a look at the ICE 180, MLB, UMB...etc.

So with the tranformer my PI network tuner should resonate better (?)

I plan to keep using it regardless given its recently added back to
back voltage limiting diodes between input and ground and a static
bleed off resistor for DC to ground.

This is what I have going now and it has reduced noise.

I am running a 12 guage wire from the ground banana plug of my antenna
tuner to a copper wire brade that connects my TV antenna to a ground
rod. This composes one end of my coax ground to eliminate common mode
currents.


The other end is where the coax meets my 13 meter random wire hung on
a wooden fence.

Lacking a proper ground rod, I soldered a short length of 12 guage
wire from the coax sheild to the bottom of a diced up coat hanger (16
inches in length) and stripped away the 12 guage wire sheild for the
length of the hanger segment (exposing the copper). This I plunged
into the resonably moist soil under a lylac tree.

Hodge podge for sure. But I'm on a budget till my new job starts next
week.

Thanks for all the help,

Mike