I bought a 6 foot whip and 25 ohm rheostat at radio shack and wired them up like this:
----
\ / 6 foot whip
\/
|
|
\
/
rf in ----->\ 25 ohm radio shack rheostat
/
\
|
---
/// gnd
I did some experiments with it ( off a quarter mile with my xyl reading base rig
s=meter back over 2 meters).
Here are HFPACK thread discussions about it:
----------------------
Date: Tue Dec 30, 2003 1:27 am
Subject: 1500 miles on $5 homebrew Miracle Whip
I talked to KQ6XA yesterday standing in my back yard running
an FT817 on battery power at 5 watts. I live in Juneau
Alaska. That is 1525 miles (2454.46 km)
This was on the HFPACK freq of 18.157.5 USB.
My home brew miracle whip:
6 foot RadioShack extendable whip
(I forget what this cost- you could use a tv antenna whip)
25 ohm RadioShack wirewound potentiometer (rheostat) (about $3)
A piece of circuit board to mount the pieces (junkbox)
An SO239 (female PL259) (junkbox)
A male to male pl259 (junkbox)
Circuit:
Not much to it.
Drill a hole for the center of the 239 in the circuit board
Solder the shield of the SO239 to the circuit board
Drill a hole and mount the rheostat
Center pin from SO239 goes to center connector (wiper)of rheostat.
Shield of SO239 goes to bottom connector of rheostat
Antenna goes to top connector of rheostat
make sure the antenna is insuated from the circuit board and
bind it to the circuit with some tiewraps
I can put up a picture if anyone is interested.
Comments:
The rheostat acts like an autotransformer at RF frequencies.
I am always able to get a 1:1 match on it.
It tunes so easily that I think it is part dummy load and part
autotransformer.
(Ever try using a 100 watt lighbulb as an autotuner? Ive done
it. Its a bit frightening but it works.)
I used a big electrician's wirenut as a knob for the rheostat
I can put up a picture if anyone is interested.
[Mike KL7R]
--------------------------
Mike,
Please send or post the pics for the antenna.
You could email them to me directly.
73
Fritz
WD9FMB
---------------------------
Mike, I would love to see a picture of the homebrew
miracle whip. Rusty de KC0LMS
---------------------------
Sounds like a real leaky dummy load to me.
Al, N8ARO
---------------------------
> Sounds like a real leaky dummy load to me.
I remember my elmer telling me that when he was waiting for his
novice license, he practiced sending CQ into a light bulb and
someone in the states came back to him on his light bulb!
---------------------------
>
> > Sounds like a real leaky dummy load to me.
>
> I remember my elmer telling me that when he was waiting for his
> novice license, he practiced sending CQ into a light bulb and
> someone in the states came back to him on his light bulb!
I've heard this hundreds of times but how do you actually do it?
Just pump the transmitter output into a light socket? No matching
network or resistance required? Is the bulb rating equal to the out
put of the rig, i.e. 100w rig = 100w bulb?
MarkF
K1MKF
---------------------------
> K1MKF wrote:
>> kl7r wrote:
>> ...sending CQ into a light bulb ...
> I've heard this hundreds of times but how do you actually do it? ...
No, don't do it. That was in the tube days.
Nowadays it would blow the finals. You could use a 50 ohm dummy load
though and accomplish the same trick, as coax wil radiate. My guess is
that you won't get out much on ladder line into a purely resistive load
though. There is a nice article on RF, coax, baluns, etc. In the recent
QEX.
We used a light bulb for a while but switched to a parallel fan-like
cluster of 10 500 ohm 10 watt resistors (not wire would as they would be
inductive) to give 50 ohms at 100 watts, and them immersed that in a
gallon paint can filled with transformer oil acquired from a neighbor at
the power company so it could take more power. I remember seeing it
bubbling out during a VHF tune-up with an old tube-type Motorola X53GGT
2m transmiter. I shudder now to think what was in the transformer oil,
but it was the early 1970's so I don't know for sure.
Leigh
WA5ZNU
-------------------
>
> I've heard this hundreds of times but how do you actually do it?
> Just pump the transmitter output into a light socket? No matching
> network or resistance required? Is the bulb rating equal to the
Yes thats what I did one day. Ran coax to a litebulb socket.
It is a bit frightening on a solid state rig. The swr spikes
up (ie a dead short) when you first start transmitting
but when the filament starts glowing white after a second or so
the swr drops down to 1:1.
I paralled my loop antenna across the light bulb and
was able to use it on multiple bands with the same effect.
I quit after a night of experimenting and proving to the
guys on my favorite net that it worked.
I dont think I would recommend this if you have a solid
state rig. A tube type final can handle the swr spike a bit
better than a solid state rig.
KL7R
------------------
> I've heard this hundreds of times but how do you actually do it?
> Just pump the transmitter output into a light socket? No matching
> network or resistance required? Is the bulb rating equal to the out
> put of the rig, i.e. 100w rig = 100w bulb?
> ....
I can't imagine sending in much power with solid-state finals, even
with a tuner, as its a very small inductor in series with a resistor
that changes dramatically in value with applied power and time. I've
only ever heard of it done with tube rigs to the point of lighting the
bulb. At low enough power, though, one could combine a few bulbs in
series and parallel to get to 50 ohms with some inductive reactance,
and be fairly stable as long as you never put out enough current to
raise the temperature of the filaments significantly.
73,
tim
-----------------------
> I can't imagine sending in much power with solid-state finals, even
> with a tuner, as its a very small inductor in series with a resistor
> that changes dramatically in value with applied power and time.
Thanks for all the replies. I think I'll just stick with my trusty
MFJ dummy load and some cheap coax.
MarkF
K1MKF
--------------------
Hey Mike we have been kidding you about the Dummy Load Antenna, however, it
does sound simple and interesting.
Could you run some field strength testing sometime at different SWR
settings. A 1:1 SWR may be all resistive from the rheostat and you may
radiate more RF at a greater SWR. Just a thought.
Al. N8ARO
--------------------------
Al I am planning to do some A/B testing of the rheostat MW
and a same sized whip with a BLT tuner. The idea of using a
field strength meter is a good one. Maybe my wife who is
also a ham could be my "field strength meter". That way I
could go out of the near field of the antenna and see how
they act a bit further out.
A note here to all:
I must give credit to Bonnie KQ6XA.
She made a version of this antenna before I did that did pretty
well in the antenna shootouts.
The original article in QST mentioned using a rheostat body
as the wiper which contacted the autotranformer toroid.
I thought since the rheostat is inductive at rf that you could
just use the rheostat itself and so went to radio shack to
look for wirewound pot.
Before I got my version of the MW built, I found Bonnie's version
in the HFPACK shootouts which also uses a rheostat -just a much
larger one. Interestingly her version (called a Wonder Whip)
did pretty well in the 2002 hfpack antenna shootout - only down
half an S unit from a full sized vertical.
Check out:
http://www.qsl.net/hfpack/antennas/shootoutvertical2002.html
-------------------
I went out tonight (PS it is 5 above here gorgeous and clear).
I took out the radioshack rheostat MW and a NORCAL BLT with a
duplicate whip attached to one leg of the norcal blt and the other
leg of the norcal blt shorted to ground.
I walked about a quarter of a mile away from our house and setup
both antennas (the 6'MW with a 25 ohm rheostat and BLT/6'whip)
I had my wife via 2 meters read me back S-meter readings on my
base rig.
The Rheostat mw did much worse than the BLT whip.
S6 vs S1 !
So I guess it is mostly dummyload :(
I did try a near field test of SWR vs field strength.
The fieldstrength seemed greatest when the SWR was "2 bars" [on my f817].
After dropping the swr to zero, if you kept turning the pot
fieldstrength would drop. In the far field there was no noticable
difference in the SWR setting vs field strength.
So it appears that if you get a broad null, the best position
of the pot should be at where it just hits "no bars" and
no farther.
A lower ohmage rheostat would have less resistive loss
but I dont know where to find a low value rheostat locally.
Hey, The good news is that it means I was 30 db down from a 6 foot
whip and a BLT and still was able to talk 1500 miles.
So the plan now is to remove the nichrome wire and replace
with #30 magnet wire and see what happens. My radio shack
store has 3 or 4 more of the 25 ohm rheostats in case there
no improvement or if I crater the current one.
Mike
KL7R
---------------------------
I pulled apart the rheostat and pulled off the nichrome wire
and wrapped the core with #30 magnet wire then sanded the inside
of the wire to expose the bare wire.
The receiver peaks are much sharper now
The transmitter SWR dips are much sharper also.
It seems to need a counterpoise more now.
The resistance of the magnet is about 1.5 ohms so this
version must be nearly completely inductive.
Ill do A/B tests tmw..
--------------------------------------------------
Mike, thanks for sharing all you fun and hard work with us.
Al, N8ARO
----------------------------
I did the A/B tests this morning
After replacing the nichrome wire with #30 magnet wire
the tests showed an improvement but still not as good
as a BLT tuner and whip.
The field strength readings this time were:
BLT/Whip S9
Rheostat body with #30 wire and whip S7
So the homebrew mw is down 12 db instead of the previous 30 db
Or saying it another way:
Replacing the nichrome wire with magnet wire was worth 18db or
3 S-units.
I think Ill quit at this point.
I might try putting an L network at the base of the whip
sometime and see if it comes close to the BLT/whip combination
Thanks for the feedback,
Mike KL7R
---------------------------
I will say that I have used it for QS0s to other parts of Alaska (600+ miles)
and have talked to a fellow walk-about in seattle who was running an ft817 and
a MW. And , as I mentioned at the beginning of this, I did QSO with California
from here in Alaska standing in my back yard with the rig and antenna in one
hand and the mic in another.
I keep the miracle whip in my "qrp bag" I use it (and a 1 foot square
shielded loop) for signal DF-ing. It is cool to take along to the ham club and
use for testing and bragging rights but I dont use it every day. I am a fan
of big wire antennas (big wire loop antennas actually). I actually use an
end fed piece of wire (88 feet) and a BLT tuner for most of my in the field qrp
work.
73 Mike