# Shortwave random-wire antenna question

D

#### Dave

Jan 1, 1970
0
I recently hooked up a thirty-six foot (plus or minus a couple of feet)
piece of four-stranded wire with alligator clip to the internal "whip"
antenna of my portable shortwave receiver, for the extra performance such a
device offered. It works so well that I now cannot usually use my "DX"
setting because of all the background noise (sounds like hundreds of other
broadcasts vying for attention.) I don't know the frequency source of all
this background noise, but would like to filter out as much of it as I can.
One manufacturer of a similar "wind-up" antenna adds a capacitor to the wire
in order to lower the resonance frequency of the wire. If I were going to
try something similar (adding a capacitor, in series) in an attempt to bring
the resonance of the wire down into the 30 MHz range, what size (roughly)
capacitor should I use? Should I just try a few with different ranges, or
does anyone here have any suggestions?

TIA

Dave
[email protected]

M

#### Matt J. McCullar

Jan 1, 1970
0
Do you have some filtering built in to your radio? Some of the better rigs
have noise blanking, AGC, extra RF amplification (which can be bypassed),
frequency shift/width, etc.

T

#### Telamon

Jan 1, 1970
0
Dave said:
I recently hooked up a thirty-six foot (plus or minus a couple of feet)
piece of four-stranded wire with alligator clip to the internal "whip"
antenna of my portable shortwave receiver, for the extra performance such a
device offered. It works so well that I now cannot usually use my "DX"
setting because of all the background noise (sounds like hundreds of other
broadcasts vying for attention.) I don't know the frequency source of all
this background noise, but would like to filter out as much of it as I can.
One manufacturer of a similar "wind-up" antenna adds a capacitor to the wire
in order to lower the resonance frequency of the wire. If I were going to
try something similar (adding a capacitor, in series) in an attempt to bring
the resonance of the wire down into the 30 MHz range, what size (roughly)
capacitor should I use? Should I just try a few with different ranges, or
does anyone here have any suggestions?

Since you have cross posted to sci.electronics.basics lets try to look
at this logically and as non technically as possible.

The are two things that you need to accomplish to hear a station on your
radio in the way of signal strength.

1. The signal must large enough for the radio to amplify it and
reproduce it at the speaker.

2. The signal must be stronger than the noise floor of the radio and any
external noise the antenna picks up by some margin over the station you
want to hear. Usually this is something like 10 dB.

You can't do anything about the noise floor of the radio unless you want
to modify it. The basic sensitivity of the radio is a decision you made
when you bought it.

That leaves the antenna. What you did was to put up the most basic type,
which is called a Marconi or common mode antenna. For a simple antenna
it is about as non-selective as you can make hence the noise level is
radio itself may be generating some of the noise. Portables are designed
to be sensitive and simple so they can't handle much signal. A strong
signal out or in band could be causing you additional trouble.

Whether 36 feet of wire is to much or not depends on where you live but
that it is picking up everything well including lots of locally
generated noise.

That is the basically where you are at.

(stations) you want to hear without hearing noise from other electrical
appliances or stuff out of band.

You need a more complex antenna design that will not pick up as much
noise as the signal you want to hear. Noise is on all frequencies and
comes from all directions.

A more complex antenna design can do things like:

1. Limit the direction it picks up signal or noise. You can benefit from
this by pointing the antenna at the signal you want or conversely
attenuating a noise source.

2. Changing the type of energy the antenna picks up. The antenna type
determines whether it picks up common mode or differential mode.

3. The antenna type also determines whether it is sensitive to the
electric, magnetic fields or both.

4. The antenna type also determines the band or bands of frequencies it
will pick up well.

All the above will limit the total amount of noise energy it will
present to the radio so it has less to deal with. Basically you use the
antenna design to preselect the signals you want to pick up. The
downside of this is short wave covers a wide range of frequencies so you
will need more than one antenna. For some type of resonant antenna the
smallest number of antennas you need are two and better would be three.

To get started with a more complex antenna and to see if you are really
whip antenna. Use a station on a high band (smaller antenna) during the
daytime.

Make a simple resonant antenna like a dipole cut for that frequency
connected to a coax and determine how to connect the coax to your radio.
If it is a portable radio try operating on the batteries as some of the
wall wart power supplies are noisy or noise on the house wiring is being

Now to test the antenna to see if it really helping you can disconnect
it from the radio and extend the radios whip antenna and collapse it
again reconnecting the external antenna to see which works the best.

You can put the external antenna outside away from noise generating
electrical equipment or switch them off.

Once you have a dipole making an improvement on weak signals you can
make other antenna types and antennas for other frequencies.

There are plenty of antenna sites on the web and ideas on finding local
noise sources.

C

#### cornytheclown

Jan 1, 1970
0
Dave said:
I recently hooked up a thirty-six foot (plus or minus a couple of feet)
piece of four-stranded wire with alligator clip to the internal "whip"
antenna of my portable shortwave receiver, for the extra performance such a
device offered. It works so well that I now cannot usually use my "DX"
setting because of all the background noise (sounds like hundreds of other
broadcasts vying for attention.) I don't know the frequency source of all
this background noise, but would like to filter out as much of it as I can.
One manufacturer of a similar "wind-up" antenna adds a capacitor to the wire
in order to lower the resonance frequency of the wire. If I were going to
try something similar (adding a capacitor, in series) in an attempt to bring
the resonance of the wire down into the 30 MHz range, what size (roughly)
capacitor should I use? Should I just try a few with different ranges, or
does anyone here have any suggestions?

TIA

Dave
[email protected]

Antenna basics with formulas

http://www.electronics-tutorials.com/antennas/antenna-basics.htm

you may also want to do a search on "antenna tuners"

B

#### Bob Myers

Jan 1, 1970
0
Dave said:
One manufacturer of a similar "wind-up" antenna adds a capacitor to the wire
in order to lower the resonance frequency of the wire. If I were going to
try something similar (adding a capacitor, in series) in an attempt to bring
the resonance of the wire down into the 30 MHz range, what size (roughly)
capacitor should I use? Should I just try a few with different ranges, or
does anyone here have any suggestions?

to be a lack of selectivity in the radio (in other words, it
is accepting signals over too wide a bandwidth, so you hear
not only the station you're interested in, but those "to either
side" as well. Having the passband too wide also makes
for more noise in general. There are filters that can be
bet may be to simply look for a better receiver.

Bob M.

R

#### Rich Grise

Jan 1, 1970
0
Bob Myers said:
to be a lack of selectivity in the radio (in other words, it
is accepting signals over too wide a bandwidth, so you hear
not only the station you're interested in, but those "to either
side" as well. Having the passband too wide also makes
for more noise in general. There are filters that can be
bet may be to simply look for a better receiver.

Not necessary at all. With a little studying of how antennas work,
Dave could build an antenna tuner and preselector all rolled into
one, with only a few parts.

For specifics, that's a homework problem and I'm personally
currently engaged in a project much like this; I'll post when
I have something a little more concrete.

Cheers!
Rich

D

#### Dave

Jan 1, 1970
0
Rich Grise said:
going ranges,

Not necessary at all. With a little studying of how antennas work,
Dave could build an antenna tuner and preselector all rolled into
one, with only a few parts.

For specifics, that's a homework problem and I'm personally
currently engaged in a project much like this; I'll post when
I have something a little more concrete.

Cheers!
Rich

Thank you, Rich. That is just what I have decided I need. Where are you
finding your information on building such a device?

Dave

J

#### -=jd=-

Jan 1, 1970
0
{snippage}

Thank you, Rich. That is just what I have decided I need. Where are
you finding your information on building such a device?

Dave

I've no interest re-inventing the wheel, so if you come up with a design
or have links to the design(s) you've settled on - and if you are
agreeable to it - please share.

Thanks!

-=jd=-

R

#### Rich Grise

Jan 1, 1970
0
-=jd=- said:
I've no interest re-inventing the wheel, so if you come up with a design
or have links to the design(s) you've settled on - and if you are
agreeable to it - please share.

Well, I was just basically thinking of an ordinary antenna tuner,
which can be as simple as one capacitor or one inductor, or various
combinations, depending on what kind of impedance you're seeing
at the feed point.

The seat-of-the-pants way to do it is get a variable cap and
variable inductor in a range appropriate for the freq, and just
stick them in various arrangements and play with the tuning

If you want to be scientific aboutg it, you might find something here:

Have Fun!
Rich

P

#### Private

Jan 1, 1970
0
Dave,

I have been in a very similar situation. I used almost an identical
sufficent filtering in this case (Antenna is likely working great). I
would start by ensuring the antenna has a good ground connection to

An antenna tuner/preselector would defantely help. Purchasing a new
or used amateur radio tuner (i.e. MFJ) 1.8Mhz-30Mhz would be the
easiest option. Adding capacitors separetely could work, but from
experience is a challenge to achive an workable solution for all
frequencies.

Homac

J

#### -=jd=-

Jan 1, 1970
0
{snippage}

The seat-of-the-pants way to do it is get a variable cap and
variable inductor in a range appropriate for the freq, and just
stick them in various arrangements and play with the tuning

If you want to be scientific aboutg it, you might find something here:
22antenna+tuner%22

Have Fun!
Rich

The variable inductor is my stumbling block. I want that continuously
variable functionality, but without dropping $100 -$200 for a new/used
inductor. I'm thinking about trying to build one - but that's all I'm
doing... just thinking...

-=jd=-

D

#### Dave

Jan 1, 1970
0
The antenna does indeed seem to be working great, and the radio is not
operating too shabily either as it is only the "background noise" I am
trying to reduce. I hooked a 100 mH RF choke up to it with good results,
and am planning on adding another one or two similar devices in an effort to
cut down on higher frequency interference. Question: how would I ground
this antenna? I have a grounding rod right outside the window, but don't
know what to hook it too. The negative battery terminal? This radio does
have an external antenna input, but that has a plastic ring around the
outside. Open to suggestions. And thanks for the input RE purchasing a
new/used tuner.

dave
[email protected]

D

#### Dave

Jan 1, 1970
0
Bob Myers said:
to be a lack of selectivity in the radio (in other words, it
is accepting signals over too wide a bandwidth, so you hear
not only the station you're interested in, but those "to either
side" as well. Having the passband too wide also makes
for more noise in general. There are filters that can be
bet may be to simply look for a better receiver.

Bob M.

Hello Bob,

The selectivity doesn't actually seem to be too bad, as it is only the
weaker signals that I have trouble digging out of the hash and trash. It
does seem to be picking up some out-of-band signals, but they are very, very
weak. As I posted in another message, a 100 microhenry RF choke cut that
stuff out considerably, and I am planning on adding another 100 or 200
microhenry RF choke to see if that helps make the slightly stronger signals
a little more discernable. Next on the worksheet is an antenna tuner of
some sort.

Thanks all,

Dave
[email protected]

Y

#### Yodar

Jan 1, 1970
0
DAVE : Wind yer coil of at least a hundred turns of magnet "war"
around a 1" PVC pipe segment (when I was a KID I used a lacquered
toilet paper core) and take taps off it every 2-5 turns (arbitrary)
and use the selector switch to derive signal from the taps that give you
the best performance (it will vary per band) The capacitors are from
old garage sale radios and may not even be needed

certain frequencies can be canceled out by selection of signal
help you tune it in more making a tank circuit that resonates with the
frequency you're "working"

I did perhaps the same thing by winding 100 turns of magnet war on a
salvaice ring torid core from a ttransistor powwer supply off a dead
computer...The antenna went to 1 end of this 100 turns and the other end
was grounded. On top this hundred turns I wound 25-40 turns and ran on
end of THAT winding to my radio's antenna IN connector and the other end

You have just made a 4:1 balun...which all in one almost does what the

Or you can do the quick and dirty trick with a TV 300 ohm to 75 ohm
coupler, connecting the 300 ohm end to the antenna and ground as above,

You wont notice the difference

Yodar

The antenna does indeed seem to be working great, and the radio is not
operating too shabily either as it is only the "background noise" I am
trying to reduce. I hooked a 100 mH RF choke up to it with good results,
and am planning on adding another one or two similar devices in an effort to
cut down on higher frequency interference. Question: how would I ground
this antenna? I have a grounding rod right outside the window, but don't
know what to hook it too. The negative battery terminal? This radio does
have an external antenna input, but that has a plastic ring around the
outside. Open to suggestions. And thanks for the input RE purchasing a
new/used tuner.

dave
[email protected]
...

Y

#### Yodar

Jan 1, 1970
0
I just tried to send a post with a schematic for a simple antenna tuner
and the diagram was cut out...how do I add it?

Yodar

D

#### Dave

Jan 1, 1970
0
Maybe post it to alt.binaries.schematics.electronics. That is set up to
take diagrams.

Dave
[email protected]

C

#### CW

Jan 1, 1970
0
For many years, it was common practice to use a lead and aligator clip in
place of a roller inductor. Continuously variable and cheap.

C

#### CW

Jan 1, 1970
0
You don't in this newsgroup (shortwave). This is a text only group.

J

#### -=jd=-

Jan 1, 1970
0
For many years, it was common practice to use a lead and aligator clip
in place of a roller inductor. Continuously variable and cheap.

rod?), positioned so that the bead can be lifted just slightly, then slid
along the coil - but when you release the bead, there's a bit of tension
pressing it against the coil. I thought about the gator clip method, but
I'm
thinking it might be easier to fool with if I had something I could adjust
without taking my focus off the radio.

Then again, alligator clips and leads are a more widely available than the
pseudo roller inductor parts rattling around in my head...

-=jd=-

F

#### Frank Dresser

Jan 1, 1970
0
-=jd=- said:
rod?), positioned so that the bead can be lifted just slightly, then slid
along the coil - but when you release the bead, there's a bit of tension
pressing it against the coil. I thought about the gator clip method, but
I'm
thinking it might be easier to fool with if I had something I could adjust
without taking my focus off the radio.

Then again, alligator clips and leads are a more widely available than the
pseudo roller inductor parts rattling around in my head...

-=jd=-

I'll suggest you try the alligator clip and wire first. It will be alot
easier to do any modifications. Also, you might find the whole experiment
isn't particularly helpful in your situation. If so, you might as well find
out right away.

Frank Dresser

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