SWL longwire
Scott Ryan
I use a 75' random longwire for shortwave listening. When I first put
it up I simply feed it with insulated wire with good results.
To improve it in regards to noise, I strung about 45' of Belden RG 58
coax as a feed line. I soldered the center conductor to the longwire,
left the shield "open", terminated the receiver end with a PL259, then
connected the coax to the receiver's 50 ohm tap. A lightning arrestor
is also inline.
My problem is that this setup doesn't seem much, if any, better then the
insulated wire it replaced. It would seem that local noise would be
less & the signal would be stronger with coax. My receiver is a SW8
which has both low "Z" & high "Z" antenna inputs.
I wonder if a longwire balun would help match the impedance & provide a
better signal?
Sander Schimmelpenninck
Scott: I think you need to ground the shield you left "open". You can connect
it to your antenna tower if that is what you use. It looks to me as if your
ungrounded coax just works as a wire.
Sandy
Oakville, Ontario
In article <32B40B...@fyiowa.infi.net>, Scott Ryan <cr...@fyiowa.infi.net>
wrote:
John Doty
In article <32B40B...@fyiowa.infi.net> Scott Ryan <cr...@fyiowa.infi.net> writes:
> I use a 75' random longwire for shortwave listening. When I first put
> it up I simply feed it with insulated wire with good results.
>
> To improve it in regards to noise, I strung about 45' of Belden RG 58
> coax as a feed line. I soldered the center conductor to the longwire,
> left the shield "open", terminated the receiver end with a PL259, then
> connected the coax to the receiver's 50 ohm tap. A lightning arrestor
> is also inline.
>
> My problem is that this setup doesn't seem much, if any, better then the
> insulated wire it replaced. It would seem that local noise would be
> less & the signal would be stronger with coax. My receiver is a SW8
> which has both low "Z" & high "Z" antenna inputs.
>
> I wonder if a longwire balun would help match the impedance & provide a
> better signal?
The way the coax shield works is that it carries a current that
cancels the external electromagnetic field, preventing it from
reaching the inside of the shield. Leaving the shield open circuited
as you have gives this current nowhere to flow, so the shield
effectiveness is drastically reduced.
The key to effective coax use with an unbalanced antenna like a
"random wire" is grounding the shield, especially at the antenna
end. Your coax should end at a ground stake, and the antenna wire
should be brought down to ground level to connect to it. With a
balanced antenna, grounding the coax shield may also inprove
performance (but it's not as important as in the unbalanced case).
The reason that you need to bring the antenna to ground is that
you can't practically bring the ground to the antenna! The reactance
of a wire depends on its shape, diameter, and the configuration of
nearby conductors. However, for an uncoiled wire shorter than about
1/10 wavelength, the reactance is inductive and may be estimated
within a factor of two or three by 10 ohms/meter/MHz. Thus, a 1
meter ground wire at 10 MHz will have a reactance of order 100 ohms
(pretty mediocre). A ground wire must be *very* short to be effective
at HF.
Without a matching transformer (like the so-called "Magnetic Longwire
Balun"), your antenna may have deaf spots at antiresonant frequencies,
where the wire length is a multiple of 1/2 wavelength. A matching
transformer can supress the antiresonances (it also supresses the
resonances, which is normally no problem for HF receiving). Don't
however reward ignorant salesmanship by buying a "Magnetic Longwire
Balun": the ICE matching transformer is better built (by people
who actually understand how it works) and cheaper. Alternatively,
making your own is not difficult.
See "Low Noise Antennas" at "http://www.grove.net/~badx/" for more
details.
--
John Doty "You can't confuse me, that's my job."
j...@space.mit.edu
David Rickmers
On Sun, 15 Dec 1996 08:29:07 -0600, Scott Ryan <cr...@fyiowa.infi.net>
wrote:
>I wonder if a longwire balun would help match the impedance & provide a
>better signal?
Yes and no. Palomar has an MLB for $40 but they also have a less
elegant but beefier 12:1 "UnUn" for $10.00 less. I recommend calling
them at 619 747 3343 and requesting a catalog. They also have an
email address at Compuserve, but my enlarged testicles prevent me from
finding that number.
dr
tv...@mailbag.com
Scott Ryan wrote:
>
> I use a 75' random longwire for shortwave listening. When I first put
> it up I simply feed it with insulated wire with good results.
>
running the wire straight into the receiver, no splices in the wire. Run
it to the 500 ohm input, or the high impedance input, depending on your
rig.
> To improve it in regards to noise, I strung about 45' of Belden RG 58
> coax as a feed line. I soldered the center conductor to the longwire,
> left the shield "open", terminated the receiver end with a PL259, then
> connected the coax to the receiver's 50 ohm tap. A lightning arrestor
> is also inline.
>
You should not be using coax for this type of antenna setup.
> My problem is that this setup doesn't seem much, if any, better then the
> insulated wire it replaced. It would seem that local noise would be
> less & the signal would be stronger with coax. My receiver is a SW8
> which has both low "Z" & high "Z" antenna inputs.
>
> I wonder if a longwire balun would help match the impedance & provide a
> better signal?
If you want a great shortwave antenna, and use coax for the lead in,
make a dipole antenna. A dipole antenna is fed in the center. Contact
Amateur Electronic Supply at 800.558.0411 and buy a W2DU-HF 1:1 choke
balun for $26.00. To find the length of wire you need to make a half
wave dipole, use this formula: 468/frequency in mhz = halfwave dipole in
feet.
Example: 468/3.900 mhz = 120 feet
My dipole is 120 feet, I find it works great for all shortwave bands and
AM broadcast band as well. I picked 120 feet because I use it to
transmit in the 80 meter band. You put the balun in the center, so in
the above example you have 60 feet of wire on each side of the balun.
For safety, never run your antenna over or under power lines.
Leonard Weldon
Ref: Short-Wave antennas....
Before you make a decision...make sure you look at the
specs, and try to take a look at the Alpha Delta DX-SWL-S
60 ft. "Sloper" antenna. The quality of build is great and
it uses broadband low-QRF choke-resonators for multi-band
frequencies...works from Broadcast up to 30mhz. Does
great on Amateur frequencies..75/40/20/10 meters.
It is actually designed for Amateur use when cut as the (40 ft.)
amateur frequencies model. The sloper offers some advantages when
finding a mounting place...offers great advantages over a
dipole on DX shortwave. Having fooled around with various dipoles
cutting them, various feed line efforts, etc...I tried the "Sloper"
and it worked much better in terms of mounting, and seemed to
be equal to any other antenna I put up...much quieter than a long
wire, yet seems to pull out DX in the direction it favors...worth
looking at.
I'm using it with a Kenwood R-5000 receiver.
If I remember right..the Alpha Delpha costs in the vicinity
of $60.00. Consider RG8 coax if possible...the antenna is
end-fed..and provides the SO/PL-259 connections.
Take a look at it!
Leonard...
lcw...@worldnet.att.net
_________________________________________________________
On Wed, 18 Dec 1996 01:06:41 -0800, tv...@mailbag.com wrote:
:
Len Anderson
TV>From: tv...@mailbag.com
TV>Subject: Re: SWL longwire
TV>Scott Ryan wrote:
TV>>
TV>> I use a 75' random longwire for shortwave listening. When I first put
TV>> it up I simply feed it with insulated wire with good results.
TV>>
TV>Scott, for shortwave reception, I think you will find it hard to beat
TV>running the wire straight into the receiver, no splices in the wire. Run
TV>it to the 500 ohm input, or the high impedance input, depending on your
TV>rig.
Splices are perfectly acceptible if there is good contact. The
impedance of an isolated single wire to the AC power "ground" will
vary over great extremes versus frequency.
TV>> To improve it in regards to noise, I strung about 45' of Belden RG 58
TV>> coax as a feed line. I soldered the center conductor to the longwire,
TV>> left the shield "open", terminated the receiver end with a PL259, then
TV>> connected the coax to the receiver's 50 ohm tap. A lightning arrestor
TV>> is also inline.
TV>>
TV>You should not be using coax for this type of antenna setup.
Why not? The only thing "wrong" with that is the lack of an external
ground connection at the antenna end of the coaxial cable...primarily
for static accumulation bleed-off or arresting _outside_ the
residence. This is an "unbalanced feed" situation where the coaxial
cable outer conductor ("braid, shield") forms part of a "dipole."
The pattern is unpredictable as is the impedance seen at the
receiver. An antenna matching unit between coax and receiver will
resolve that.
TV>> I wonder if a longwire balun would help match the impedance & provide a
TV>> better signal?
No, it will (primarily) change only the magnitude of the antenna
impedance over frequency. Some bands will have more sensitivity than
other bands. The antenna tuner will take care of that.
The main advantage of using a toroidal transformer is that a longwire
can be made to ground at DC. That provides a constant path to ground
to equalize static electricity charge with nearby objects during
thunderstorms. A charge build-up can be the trigger for a lightning
"leader" the preceeds the main bolt(s).
TV>For safety, never run your antenna over or under power lines.
Quite true and a good tip. Close power lines will also induce a lot
of noise garbage in an antenna from the neighborhood's total
electrical useage...it's good to keep an antenna as far as possible
from power lines.
---
* OLX 2.1 * Just when you make ends meet, someone moves them...
John Moran
for a SW longwire..try attatching a wire to the radio hat is connected to
the air conditioning ducting..that's what I've done..works quite
well...the duct work runs the entire length of the house
--
---
JOHN/PHX (jam...@analon.com) computing & making good Chemistry!
-------
John Doty
In article <96121823...@mogur.com> len.an...@mogur.com (Len Anderson) writes:
> TV>> I wonder if a longwire balun would help match the impedance & provide a
> TV>> better signal?
> No, it will (primarily) change only the magnitude of the antenna
> impedance over frequency. Some bands will have more sensitivity than
> other bands. The antenna tuner will take care of that.
Actually, a fixed matching transformer can dramatically reduce the
wild swings in antenna efficiency that a coax fed wire antenna
exhibits. Let us calculate:
The following graphs are based on a 15 meter vertical antenna, fed
at ground level, using a conical approximation. The antenna's
characteristic impedance is assumed to be 620 ohms, which is typical
for a thin wire. For more on the conical approximation, see Chapter
8 of "Antennas" by John D. Kraus (McGraw-Hill, 1950).
The first graph is for an antenna fed directly from 50 ohm coax.
The horizontal axis is the frequency in MHz, the vertical axis is
the mismatch loss in dB. The well known "quarter wave" resonances
near 5, 15 and 25 MHz are visible as sharp peaks where the mismatch
loss closely approaches zero.
#
###########################################################################
# ## ## ##
# # # 10 ## 20 ## 30
# # # # # # #
# # # # # # #
# # # # # # #
-5 # # # # # # #
# # # # # # #
# # # # # # #
# # # # # # #
# # # # # # #
# # # # # # #
-10 # # # # ## # ##
# # # # ### # ####
# # ### # ###### ### ##
# # #### ## ##
# # #####
# #
# #
-15 # #
# #
# #
# #
# #
#
The second graph assumes a matching transformer with a 9:1 impedance
ratio at the feedpoint, presenting the antenna with a load resistance
of 450 ohms. At most frequencies, the mismatch losses are considerably
lower for this case. The variation in the mismatch loss is also
reduced:
#
# #######
-3 # #### ##
# ## ## ##
# ## # #
# ##### # ## #
-4 # ## ## # # #
# # # # ## ##
# # # # ####
# # # #
-5 # # # #
# # ## #
# # ####
# #
-6 # #
# #
# #
# #
# #
-7 # #
# #
# #
############################################################################
# #
# # 5 10 15 20 25 30
# #
Well, so what? In the absence of interference, the signal to noise
ratio is the main determining factor for the audio quality of the
signal. The mismatch loss affects both signals and noise, so if
the receiver was noiseless the losses would not affect the signal
to noise ratio. Real receivers, however, are not noiseless: if the
loss is too high, receiver noise will become dominant, and overally
system sensitivity will suffer.
The following results assume cosmic noise of 29 dB above thermal
at 10 MHz, declining with increasing frequency at -23 dB per decade.
No man made or atmosperic noise is assumed. I assume a receiver
noise figure of 10 dB.
First, here is the signal to noise impact of the mismatch losses
for a 50 ohm coax feed without a transformer:
#
############################################################################
# ## ## ####
# 5 ## 10 # 15 20 ## 30
# ### # # ###
# ## # # # #
-1 # # # # # #
# ###### # # #
# ## # # #
# # # #
# # # #
-2 # # # #
# # # #
# ## # #
# # # #
# # # #
-3 # ## ## #
# #### #
# #
# #
# #
-4 # #
# #
# ##
# #
-5 #
Losses in signal to noise of 3-5 dB are likely to be noticeable.
The largest impact is in the quiet bands above 15 MHz.
On the other hand: the loss in signal to noise with a 450 ohm feed
is much smaller:
#
##########################################################################
# ###########
# 5 ####10 15 20 25 30
# ##
-0.2 # ###
# ##
# #
# ##########
-0.4 # ###
# #
# ###
# #
-0.6 # #
# #
# #
# #
-0.8 # #
# #
# ## ###
# ##### ###
-1 # ## ###
# #
# #
# #
-1.2
You are unlikely to be able to notice losses in signal to noise in
this range.
The results depend on the assumptions. A real longwire isn't usually
vertical: this tends to degrade its performance a bit at the low
frequency end, while improving it at high frequencies. This is
good, because in the model the signal to noise is declining as the
frequency increases: the increase in performance cancels part of
this.
No man made or atmospheric noise is included. If they are significant,
the precision of the match becomes less critical. Man made noise
can be significant at any frequency, but atmospheric noise is more
significant at the lower frequencies.
A receiver noise figure of 10 dB is mediocre for a solid state
receiver or a tube receiver with a triode RF amplifier. Tube
receivers with pentode RF stages may be a bit worse than this, and
something like a Hallicrafters "Sky Buddy" (no RF stage, pentagrid
converter) might have a noise figure >30 dB. The better (smaller)
the noise figure, the less you have to worry about matching. Sky
Buddy owners will want to tune their antennas very carefully.
I haven't included cable losses here. These are not terribly
important unless you're using an ATU at the receiver end. If you
are, using a fixed transformer to get the match roughly right at
the antenna end will reduce the cable losses, because cable losses
increase with increasing SWR.
My own experience concurs with the results of this theoretical
analysis (or I wouldn't be writing about it: I'd be trying to figure
out what was wrong!). I have experienced "deaf bands" with coax
fed antennas lacking matching transformers, but my transformer-fed
antennas work well across the HF spectrum (and even down to longwave).
I don't bother with an ATU.
See "Low Noise Antennas" at "http://www.grove.net/~badx/" for more
on matching transformers and on keeping conducted noise from
contaminating a "longwire antenna".
David Rickmers
On 23 Dec 1996 19:49:01 GMT, j...@space.mit.edu (John Doty) wrote:
Right on...thanks John. It should also be pointed out that an
unbalanced transformer in the antenna system provides an excellent
means of shunting "static" electricity (the kind that zaps 2010s) to
ground.
dr