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DX From Your Apartment Window
David Dalton (K9WQ)
on
March 19, 2004
View comments about this article!
10 pounds of copper and an ounce of vacuum:
DX from your apartment window
Magnetic loop and a Jennings vacuum variable capacitor.
This particular 1200 pF capacitor has the range to make the loop usable
for
transmitting on 40 to 15 meters. Reception is possible as low as 3.6
MHz.
Yes, you can work DX from a small antenna in your apartment window. And
you can build your own antenna to do this -- a magnetic loop.
Can you compete with the big guns on SSB with their towers and
amplifiers? No, though you'll certainly be heard on SSB. But especially
if you concentrate on the digital modes -- CW, PSK31, and RTTY -- you
can do a lot with a modest 100 watts and a magnetic loop.
I'm a newbie. If I can do it, you can do it. I was first licensed in
August 2002. I upgraded to General in February 2003 and to Extra in
April 2003. I don't know why I waited so long to get my ham license.
I've listened to short wave since I was a boy, and when I gave up on my
first attempt to learn Morse code at the age of 12, I thought it was
impossible. There was no one around then to tell me that it was about
learning the sound, not about memorizing dots and dashes. Like a lot of
people, I got deeply involved in digital in the early '80s. I found my
way to San Francisco in 1991, and I was at ground zero for those
amazing dot-com digital days.
After the dot-com bubble burst, and especially after the events of
Sept. 11, 2001, rocked our view of the world, I suddenly realized that
digital technology had not -- and never can -- make analog obsolete.
Radio is a beautiful, natural, eternal phenomenon. Computers are just
dumb machines. Computer programming languages are artificial and fairly
easy to learn, whereas the complex equations that describe natural
phenomona such as radio are daunting, even to smart people.
I actually know smart people who don't seem to realize that their
digital cell phone depends on old-fashioned analog radio. They still
think, as I once did, that because so much analog technology was made
obsolete by the digital revolution that we somehow don't need analog
anymore. But we live in nature. Analog will always be with us.
Sure you need computers to get a robot to Mars. But the navigation, the
communication, the propulsion: underneath, it's all forever analog.
Digital: Been there, done that. Analog: It's the new digital.
After I got my General, I was eager to get on HF. I did a lot of
reading on antennas, and it became clear that a magnetic loop offered
the best hope for my situation. I live on the fifth floor of a large
concrete and steel building. I have large north-facing windows, and one
of those windows is in the same room as my radios.
There's a lot of good information on magnetic loops on the Web. It's
easy to search for, but I'll include some links at the end of this
article. Also, Section 5 of the ARRL Antenna Book contains good
information on the theory and construction of magnetic loops. If you'd
like to build a loop, the experts have made a lot of good information
available. I am not an expert, though I've learned some things after a
year of experimenting with magnetic loops. Probably the first helpful
thing I can do as a non-expert is to list some of the mistakes I made
and confess to some of my newbie ignorance, so that you can do better
than I did.
1. When I started, I did not properly understand the difference between
balanced and unbalanced feed lines and antennas. This is important,
because as an apartment dweller, especially on upper floors, it's
unlikely that you have a good RF ground. You probably have no room for
radials. The way around this problem is to use a balanced antenna. A
magnetic loop is a balanced antenna.
2. When I started, I was clear enough on SWR, but I did not understand
about RF on the feed line, what causes it, and what to do about it. I
did not know how, and when, to use a balun.
3. When studying for the General and Extra, there's plenty of
theoretical information on feed line impedance and antenna feed
impedance. But a practical understanding of these issues requires time
and experimentation -- with low power. Properly feeding a homemade
antenna is probably the trickiest part. Eventually you will crave an
antenna analyzer. If you can't afford one, make friends with a ham who
has one.
4. I learned from failed experiments that theory and practice go hand
in hand. I was often not quite sure whether my experiments followed
proper theory on feeds and feed lines, the use of baluns, etc. So even
though I might not have understood all the theory, there was one thing
I did understand: whether or not a particular change improved or
degraded the performance of my antenna. It was often much later that I
came to understand why a particular experimental change did or didn't
help.
And now some tips on building a magnetic loop antenna that's good
enough for DX.
1. Read, read, read. The ARRL publications are excellent. The Web is a
rich source of information, though of course it varies in quality.
2. A high-voltage air variable capacitor, even one with lossy wipers,
will get you started. But eventually you'll want a vacuum variable
capacitor. More about this later. If you buy a vacuum variable
capacitor, if at all possible buy along with it mounting brackets and
connecting hardware that fit the capacitor. Vacuum variables are
expensive, fragile, and hard to support and connect to without the
right hardware.
Air variables work, but vacuum variables work much better.
3. When testing your antenna, turn on a television and some audio
equipment 10 or 20 feet away from the antenna. If you hear scary
noises, or if the picture changes on your TV or computer monitor,
you're doing something wrong. For the sake of your neighbors and your
equipment, keep the power very low until you get the stray RF under
control. I've never had a complaint from my neighbors or damaged any
equipment. I understood the risks. It's quite possible to transmit a
clean 100 watts with no interference to nearby cable TV or audio
equipment. One hundred watts probably is the limit for apartment
operation. Keep your power as low as possible. Your neighbors need
never complain. Also, if your TV is excited over your signal, Denmark
probably is not: your precious watts are being wasted.
4. Use G4FGQ's programs, or similar programs, to calculate the
dimensions of your loop. I found that the actual performance of my loop
is very close to what the program predicts. My loop, which is made of
1.25-inch copper pipe, with a circumference of about 9 feet, works on
40, 30, 20, 17, and 15 meters. I can listen on 75 meters, but
transmitting on such a small loop at such a low frequency is very, very
inefficient.
5. WB2RI has written an excellent set of instructions for building
magnetic loops (I'll provide Web links at the end of this article).
Note that there are several ways of feeding magnetic loops. WB2RI
provides instructions for a gamma feed which worked well for me. I
tried inductive feeds, with little success.
6. Build your loop right the first time. The wonderful women at Cole
Hardware in San Francisco cut my copper pipe for me very precisely.
You'll need a torch and the right solder. Aim for maximum electrical
efficiency. The total resistance of the circuit -- loop and capacitor
-- needs to be a small fraction of an ohm.
7. It makes sense, when you're still in the experimental stage, to
connect your feed and your capacitor to the loop with only screws and
clamps. But this will increase the resistance and reduce the efficiency
of your loop. Once the loop is all tuned up and working the way you
want, you're going to want all connections to be soldered, with copper
braid to connect the capacitor. However, I didn't solder the braid to
the vacuum variable capacitor. Instead the braid passes under the
silver-plated ring clamp, and then the braid and capacitor clamp are
screwed together at the end of the braid, for good measure.
8. The knob on the capacitor is your antenna tuner. The loop must be
tuned and resonant for your frequency. If you set up the feed
correctly, your loop will tune to 1:1 SWR or very close. Still, I like
to use an antenna tuner to make sure the radio always sees 1:1 SWR,
even if the loop is very slightly out of tune. This also gives me a
meter to watch, since my Yaesu FT-897 and Kenwood TS-830S don't have
true SWR meters. To tune, turn the knob on the capacitor and listen to
the radio. When the noise peaks, you're near 1:1. Make the final tuning
adjustments at very lower power until the SWR meter shows 1:1. Even at
5 watts, stay away from the antenna and the set screw on the tuning
knob. It'll bite you. The voltage at the capacitor is very high. Don't
let anyone go near it while you're transmitting.
9. Tuning with a motor is a nice idea, but I just get up and walk
across the room. Even though a vacuum variable capacitor may take 30 or
more turns to go through its range, tuning by hand is probably more
practical, because a small fraction of a turn can make a big difference
in the resonant frequency. This is probably the biggest problem with
magnetic loops: You've got to retune if you change frequencies. Again,
keep the power as low as possible while tuning.
10. At 100 watts or less 15 meters and below, you're probably OK for RF
exposure if you're on the other side of the room from the antenna. But
to be safe, do the math. I also use an inexpensive Radio Shack field
strength meter from time to time to make sure that the RF is not going
where I don't want it to go. I normally sit about 24 inches from my
radios. At 12 inches from the radio, the field strength meter will
deflect slightly. At 20 inches it does not deflect at all. Not only is
RF not good for you, if RF gets into your audio, you've got a problem,
especially if you're using delicate digital modes such as PSK31.
11. Know the rules for your building. My condo association prohibits
exterior antennas, but otherwise there are no rules against ham radio.
My antenna is just inside the window and is not noticeable from
outside. I don't use more power than I need. My building has modern
wiring, including good wiring for cable TV and good DC ground. My goal
is to not draw attention to myself and to never cause a complaint. If
it ever became an issue, I would point out to my neighbors that I'm
licensed and that I don't break the condo association's rules or the
FCC's rules. I would let them know that I've passed an ARRL emergency
communications course and ask them whether, in the event of an
earthquake or other emergency, they wouldn't prefer to have someone in
the building who has communications capability, including battery
power.
For me, the real breakthrough in my loop's performance came after a
year of using the loop, when I finally splurged for a vacuum variable
capacitor. It made a big difference in what I could hear, not just in
who can hear me. Contests provide a great opportunity for quickly
testing your antenna. During the CQ RTTY contest the weekend of Feb.
14, I easily made contacts with two stations in Japan, two in Hawaii,
and one on Aruba. As always, I did a lot of listening. I heard good DX,
including stations in Norway, Ireland, and Brazil, though I was not
able to break through pileups. East Coast stations, and most Canadian
stations, now seem easy.
To me the biggest frustation of apartment dwelling is the extreme
difficulty of getting onto 80 meters. My Jennings vacuum variable
capacitor, which cost $330 dollars (more information below), is rated
at 1200 pF and will tune my loop as low as 3.6 MHz. However, G4FGQ's
"RJELOOP2" program calculates my loop's radiating efficiency at about 2
percent at 3.6 MHz, vs. 90 percent at 21 Mhz.
Still, a calculated efficiency of 2 percent won't stop me from trying
to contact my helpful FISTS code buddy, KD5UZJ in New Mexico, on 80
meters. I'll keep trying to solve the 80-meter problem. We hams are
always looking for a challenge, right?
David Dalton, K9WQ
San Francisco
(c) 2004, David Dalton
----
Sources:
WB2RI has a great Web site on magnetic loops:
http://www.standpipe.com/w2bri/
----
G4FGQ wrote very useful programs for calculating the performance of
magnetic loops:
http://www.btinternet.com/~g4fgq.regp/page3.html
----
Vacuum variable capacitors are expensive. You may find some bargains on
eBay. If you're willing to consider an international transaction, there
are some old stock Russian capacitors listed sometimes on eBay.
Max-Gain Systems in Marietta, GA, is a good U.S. source of vacuum
variable capacitors. Used capacitors can save you a lot of money, and
Max-Gain thoroughly tests the used capacitors they sell:
http://www.mgs4u.com/RF-Microwave/
This article has expired. No more comments may be added.
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DX From Your Apartment Window
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by VE3TMT on March 19, 2004
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David,
Fantastic article. Please give more details of your loop, and antenna tuning setup. I have built many loops myself, but yours looks like a winner.
Max (ve3tmt@yahoo.ca)
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RE: DX From Your Apartment Window
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by KC8ZBV on March 19, 2004
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SWMT (single white male tech) seeking single female ham with antenna analyzer to be friends, put up antennas and long walks on the beach.
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DX From Your Apartment Window
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by KR1ST on March 19, 2004
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Very nice article David! I enjoyed it a lot.
I too have found that magnetic loops can work very well in an environment like an apartment, even at QRP levels. I worked over 80 countries with such a loop with just 5 Watts SSB. I've posted my experiences with pictures of my loop on my website at:
http://www.kr1st.com/magloop.htm
Recently I also built a magnetic loop for SWL purposes (portable shortwave radios) and wrote a How-To article about it. It's rather simple to build and contains no hard to get parts:
http://www.kr1st.com/swlloop.htm
I included a sound sample in that article so folks can hear what dramatic difference a magnetic loop can make over the built in whip antenna of a portable shortwave receiver.
Again, thanks for writing and posting your article, David!
73,
--Alex KR1ST
http://www.kr1st.com
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DX From Your Apartment Window
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by KC4ZGP on March 19, 2004
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Hello David.
Way to go! I too use a copper tubing loop. Mine is circular, ten feet circumference, one metre off the ground, and I use an air variable capacitor for tuning. I find all my ham adventure on PSK31, 14.070MHz @ fifty watts, so i needn't retune. I'd say five stories up is definitely the advantage you have over me. I shall be home about 1900UTC today March 19th. Look for me on the frequency and we'll see what our loops look like.
73
Kraus
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DX From Your Apartment Window
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by KC4ZGP on March 19, 2004
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KR1ST.
Yup that's a "Loop". The "Loop" is it! I love my "Loop". "Loop" Gooood!! If more came to their "Loop" senses, noise just might be a thing of the past. My guests hear, "have a "Loop" beer, it's imported." I too have been bitten by "The Loop". Just as PSK31 prompted me to give up all other modes, "The Loop" did the same for antennas. I must say until one actually sees and hears the difference, reluctance will manifest itself as the evil we all know--the anit-"Loop".
73
Kraus
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RE: DX From Your Apartment Window
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by KG6JEV on March 19, 2004
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Nice article! If that darned capacitor weren't so expensive, I might just build one myself.
73,
Steven
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DX From Your Apartment Window
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by KC4ZGP on March 19, 2004
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KG6JEV
Hello. I'm Tim in Warner Robins, Georgia. I use the cheaper air variable capacitors. Some say the wipers introduce losses. I've yet to experience any. My next loop project might be one with the capacitor of PCB material. Three sheets, two as the plates and the third slides between them. Or a trombone style capacitor. Many ways to make a capacitor. However trial and error until you find the right spot for resonance. Have fun!
73
Kraus
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DX From Your Apartment Window
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by WF7A on March 19, 2004
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Thanks for the great article, Dave!
Not too long ago, a ham submitted the clever idea of using a model train speed/direction controller and a stepper motor (with a worm gear drive) to use as a loop antenna tuning mechanism; a novel idea for anyone wanting to tune their loop (or other antenna) remotely. Has anybody out there incorporated this idea? If so, how well does it work?
Ciao!
Rich
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DX From Your Apartment Window
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by KG8JF on March 19, 2004
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Outstanding article! Written with a clarity that is usually not found on these type sites. Very professional
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RE: DX From Your Apartment Window
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by KA4KOE on March 19, 2004
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"I actually know smart people who don't seem to realize that their digital cell phone depends on old-fashioned analog radio. They still think, as I once did, that because so much analog technology was made obsolete by the digital revolution that we somehow don't need analog anymore. But we live in nature. Analog will always be with us. Sure you need computers to get a robot to Mars. But the navigation, the communication, the propulsion: underneath, it's all forever analog."
Amen Brother! This is excellent prose! I hope you write more in the future!
Fraternally yours,
PHILIP
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RE: DX From Your Apartment Window
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by WB2WIK on March 19, 2004
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Very nice!
I think you repeated a typo, and "WB2RI" is really W2BRI. I know Brian, he's a neighbor.
Also, there are lots of sources for surplus vacuum variable capacitors! Fair Radio Sales http://www.fairradio.com usually has them, and when they do, they are priced well and they guarantee everything they sell 100% (including all surplus and used goods).
Hams in southern CA would do well to shop at Apex Electronics in Sun Valley, who almost always has a large stock of *big* vacuum variables. I've purchased my last several from there.
Keep up the good work!
WB2WIK/6
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RE: DX From Your Apartment Window
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by AE4X on March 19, 2004
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Congratulations on this project. Great idea and great article. Wish I knew all of this during my days of apartment living.
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RE: DX From Your Apartment Window
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by VE3FFK on March 19, 2004
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Just an untried suggestion for 80m.. How about making two turns in the loop, ie twice around the window frame.
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Another great 'stealth' antenna!
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by W8KQE on March 19, 2004
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Another fantastic HF or 6m stealth or portable antenna is the end fed, PAR ELECTRONICS 'HF ENDFEDZ' antennas. Their gain is akin to a dipole, are conveniently end fed thru a small matching transformer that can be fed with regular coax/SO-239 plug, and they require no ground, counterpoises, or radials!!! Some hams even dangle them from windows, or wrap them around balconies, etc. I've used them, and they are superb! As my enthusiasm for them obviously shines through here, let me say that I am in no way affiliated with PAR Electronics. I am just a very satisfied user and wish to share info with other hams on their great antennas!
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RE: Another great 'stealth' antenna!
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by RFDOG on March 19, 2004
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Another fantastic salsa is the fresh, Trader Joes, red pepper salsa. Some hams even dangle the jars from windows, or place them around balconies, etc. I've eaten it, and it is superb! As my enthusiasm for it obviously shines through here, let me say that I am in no way affiliated with Trader Joes. I am just a very satisfied user and wish to share info with other hams on their great salsa!
sheesh...
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DX From Your Apartment Window
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by WA2JJH on March 19, 2004
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vERY NICE! I have a vertical for 10-20M on the roof of my apt. building. A few rafdials. That is it. I cannot set up a 15M inverted V.
Really need something small and compact to work 80 and 40M. This might be the one.
I am sure it will tune well on 20M with a tuner.
Many city hams are limited to "WINDOW WHIPS".
iT is an 8 foot CB mobile whip. A 14 foot wire dangles from the ground section and acts as a counter poise for
10-20M. USELESS ON 40 AND 80M!
tnx agn fer tip
73 mike WA2JJH
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DX From Your Apartment Window
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by KF6JZC on March 19, 2004
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Good article. I too was interested in magnetic loops. I suppose for some it is still the best choice. A couple of problems, though with this antenna is that it is very narrow band which is good for eliminating noise but makes it difficult to tune when you change frequencies. The other problem, and this is the reason why the tuning capacitor is so expensive is the high voltage generated across the capacitor (several thousand volts for a 100 watt transmitted signal).
Anther antenna that should be investigated is the eh (see eh-antenna.com). BTW, Ted Hart, who wrote the article in the ARRL Antenna Book on Magnetic Loops is now working on developing eh-antennas. Lloyd K. Butler is another ham developing this type of antenna. The theory behind how one of these antennas works is probably difficult for the non-electrical engineer to understand but they seem to work.
Like the Mag loop, the eh is also compact and about a noise immune as the Mag Loop. However, unlike the Mag Loop, it is more broad band but you would need an eh for each band you want to operate on. How would you like a 160m antenna that is about 10 feet long by about 10 inches in diamter.
Anyway, don't my word for it, check out the web site. You might also join the eh-antenna Yahoo group if you want to learn more. You can then directly ask both Ted and Lloyd questions about these antennas.
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RE: DX From Your Apartment Window
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by KC0JBJ on March 20, 2004
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"Not too long ago, a ham submitted the clever idea of using a model train speed/direction controller and a stepper motor (with a worm gear drive) to use as a loop antenna tuning mechanism; a novel idea for anyone wanting to tune their loop (or other antenna) remotely. Has anybody out there incorporated this idea? If so, how well does it work?"
OK, let me correct a little misconception about motors and model train controllers. The old style train controller is nothing more than a variable AC transformer and rectifier which supplies DC to the track with varying voltage. This make the train go faster or slower, and can reverse direction, because the engine is run by a DC motor, not a Stepper Motor. The difference is key, because if you apply a varying or even constant voltage to a Stepper motor, you will get not movement, except a possible quivering. You see, a stepper motor is really a series of rotary solenoids which are controlled by an electronic controller which sends pulses to each solenoid in sequence, or steps, which can control the speed and rough position of the motor. However, the control system is not perfect and sometimes steps can be missed, so absolute position is not guaranteed, unless you use a position encoder on the output shaft as feedback to the control circuit.
A better candidate for a simply actuator for the variable capacitor would be a DC motor with a planetary gear transmission which slows down the normally high-speed motor to provide slower speed and higher torque output. A battery operated power screwdriver is a good example of this kind of motor. Perhaps you could even salvage one from an old screwdrive which has worn out its batteries. Then commandeer an old train transformer (or even a new one from a cheap train set). But DO NOT use the new generation of "wireless" or "Smart" controllers. These actually send a signal to the "smart engine", which is then controlled electronically (perhaps even incorporating a stepper motor!). But these are very expensive and overkill for what you want.
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RE: DX From Your Apartment Window
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by KC0JBJ on March 20, 2004
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One question, is the vacuum casing for the capacitor a factory sealed unit or does it require periodic pump down (because no valve seal is perfect and will eventually leak down over time, just like the valve stem in your tires, except that is positive pressure, not vaccum or negative pressure)?
Do we need a vaccum pump to maintain this type of system?
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RE: DX From Your Apartment Window
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by N0SP on March 20, 2004
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KCØJBJ wrote:
Do we need a vaccum pump to maintain this type of system?
Perhaps every 300 or 400 years..
73,
NØSP
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RE: DX From Your Apartment Window
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by N3HKN on March 21, 2004
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I have a 1 meter square loop in the attic and it performs better than my GAP vertical in its FAVORED directions(2) since it has a DIPOLE like pattern off the ends. Obviously off the sides the GAP is much better. Good angle of radiation except when very close to the ground. 1 watt Pittsburgh to Florida on PSK done several times.
Another advantage is that the loop acts like a preselector Hi-Q filter. It narrow bandwidth blocks , or drastically reduces, strong signals outside of its bandwidth. Reduces any chance of signals in the Ham band, but beyond the passband of the loop, causing strange AGC action when you can't even hear them.
I use a 6 RPM motor from All Electronics coupled (insulated) to an air variable from Ocean States. It WORKS. However, noise is higher due to proximity of adjacent townhouses. On a deck, 12ft ftom the building, the noise drops and it is a great performer. However very cold wx plays havoc with the drive motor.
I use inductive coupling and have used gamma as well. Both work. I may have some capacitor loss but on receive it is not an issue and at PSK 50 watt levels a 50 watt signal works ok when reduced to 40 watts real output by losses. MUCH cheaper!
Dick N3HKN
PS: Used W2BRI page to get started.
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RE: DX From Your Apartment Window
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by WIRELESS on March 21, 2004
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I used a series of small loops and they worked very well. Some items of interest I discovered were that slightly bigger loops always work better. If a bigger loop will fit in an area, it pays to use it. Also, dual variable caps wired in 'series' bypasses the wipers and work as well as a vac cap I tried. Overall, these antennas work much better than most believe and are excellent for apartments and attics. I think alot of users of commercial products got poorly constructed versions that gave these small loops a poor reputation they don't deserve.
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RE: DX From Your Apartment Window
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by WIRELESS on March 21, 2004
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I forgot to mention that I used a single section HV cap but I clamped some very flexible braid (also tried thin copper strap) on the metal (ground)shaft and effectively bypassed the wipers. This worked very well too. Also, I tried some thick superflex wire ($3/foot) and it worked well. I found no measureable difference compared to pipe.
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RE: DX From Your Apartment Window
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by AC6IJ on March 21, 2004
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Hi Dave, this is terrific stuff and well written for sure. I have built many magnetic loop antennas and at present I have one mounted in my garage rafters. It is mounted horizontaly so that I don't need a rotor to change directions.
It is constucted of 1" copper pipe and with the circumference of 10'. I use a gamma match that is made of 1/2" copper pipe. I built the capacitor out of 1/16" aluminum sheet and is the split stator butterfly type. It is adjusted with a small DC motor that is tuned by a DC pulse modulated control box that is near my transmitter.
For a transmitter I use the Small Wonderlabs 20 rig at about 2 watts and that goes to a Ramsey 20 watt kit amplifier for the 20 watt output. My computer is a Thinkpad 760L laptop.
What could be more fun than using a magnetic loop and PSK-31. Bill
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DX From Your Apartment Window
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by AF0H on March 21, 2004
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How can we turn this into a code/no-code debate? Or how about the language used on 75-meters?
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RE: DX From Your Apartment Window
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by KA4KOE on March 22, 2004
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What would happen if we fed a fan dipole with one of these gadgets?
Just asking that what must always be asked, no matter the topic.
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DX From Your Apartment Window
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by WA2JJH on March 22, 2004
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What would happen if we fed a fan dipole with one of these gadgets?
Philip you call your self a P.E. and cannot answer your question!!!!
You get a 75M GYROSCOPIC antenna!
73 MIKE
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RE: DX From Your Apartment Window
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by KA4KOE on March 22, 2004
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No, I don't call myself one, the state of Georgia said I am one!
Besides, one has to take into account all manner of issues with respect to Doppler induced frequency shift as a result of the angular velocity in radians/second as correlated with RPM and the velocity of the radio waves in an atmosphere at a standard pressure of 760 mm HG, as it is NOT the same as C in a vacuum. This is not a simple issue on the surface, Mike.
Care to confuse the issue some more with additional techno-babble on your side? Give it your best shot!!!
PLEASE!
P
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DX From Your Apartment Window
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by W4UDX on March 22, 2004
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Very nice article. Some things to keep in mind about loop antennas:
Don't expect them to perform like a tribander.
RF voltages present at the antenna during transmit can be crazy high. Especially if the loop is small and the frequency is low. Be careful if you have young children in the house.
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RE: DX From Your Apartment Window
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by KA4KOE on March 22, 2004
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David, not to be picky, but how can you have an "ounce of vacuum"?
Good article man. Do more!!
PHILIP
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RE: DX From Your Apartment Window
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by KA1EEC on March 23, 2004
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"don't expect them to compare to a tribander"
how do they compare to a dipole? Or a random wire antenna?
Is it worth rotating one since they have two lobes of "gain"
if they are made out of copper and left outdoors, does the copper oxide (i.e. green patina) impede efficiency?
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RE: DX From Your Apartment Window
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by KC0JBJ on March 24, 2004
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<David, not to be picky, but how can you have an "ounce of vacuum"?>
Technically you cannot. The term "ounce" can be either a volumetric measure as in "fluid ounce", or it can be a measure of weight, as in 16 ounces to the pound.
Now since pressure and vacuum (negative pressure) are measured in pounds per square inch (PSI), it stands to reason that this can be easily converted to ounces per square inch. In the vernacular PSI measurments are often spoken as "Pounds of pressure", so therefore, very small measurements of negative pressure might be referred to as "ounces of vacuum".
BTW, pressure and vacuum are also often measured in inches or mmm, referring to the height of a column of water(inches) or mercury(mm)in a device known as a manometer. It measures the relative pressure between difference between two sealed vessels, between atmosphere and one sealed vessel, or between atmosphere and a sealed reference vessel. Inches of water or mm of mercury can be converted to PSI or the metric equivalent.
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RE: DX From Your Apartment Window
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by W4UDX on March 30, 2004
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You misquoted me. I said don't expect it to perform like a tribander (typical 3 element triband yagi). There is no use in "comparing" them, since the two are so different.
Any antenna that is physically shrunken to fit a small yard, window, whatever... is obviously not going to perform as well as a regular dipole, delta loop, etc... if you do want to "compare" them.
I was just giving a heads up on the antenna being a performance comprimise and RF voltage danger....
For what they are, they do well... but I would not recommend one for QRP work unless that is all you are allowed to use due to restrictions.
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DX From Your Apartment Window
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by NS6Y on March 31, 2004
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Don't let that price scare you on the vacuum variable! I see them in the sub-$100 range on ebay all the time, and they can't all be bad! Generally $50 should buy you a decent one, if anything the bigger ones seem to go for less. So, don't get discouraged, where there's a will there's a way - and those big ol' "cheese slicer" air variable caps are out there too, to get started on.
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