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

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

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

On Mon, 22 Sep 2003 19:02:21 GMT, mike wrote:

On Mon, 22 Sep 2003 16:04:04 GMT, Richard Clark
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


I guess that means it does not mattergrin

mike

Hi Mike,

In GI lingo (a variant of english) it is rendered Mox Nix (a popular
euphemism in the military meaning just as you guessed). The "Stars
and Stripes" (the Army's newspaper) used to carry a cartoon panel with
the same name.

73's
Richard Clark, KB7QHC

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

On Tue, 23 Sep 2003 05:50:23 GMT, mike wrote:

On Tue, 23 Sep 2003 03:48:02 GMT, "CW"
wrote:

Make your own transformer. Go he www.kc7nod.20m.com

Nice pics.

As for the static protection diodes. Yes, I am seeing images of local
broadcasters scattered as far as 1khz from the known frequecies.
I thought it may have been my antenna. Hmmm....might rethink the
diodes. They are 1N914 as specified.

I suspose the resistor I have across the lead in to ground should
reduce static buildup.

Did I turn my tuner into a crystal radio..grin

mike


After thinking about this a little more. What if my current impedance
mismatch between my antenna and transmission line is actually causing
reflections which are noticed by me as images on other frequencies?

Might a signal coming in on 3200 khz echo end to end back to 2300 khz?

This seems probable and the diodes may not be causing it at after all.

mike