Buck wrote:
This is an interesting antenna. It is a mobile stealth magnetic loop
antenna.

http://stealth.ae/plugins/custompage...categories=136

I watched the video and checked out the details. The antenna is
basically a loop with the roof of the car (the bottom of the luggage
rack) as the ground side of the loop, the antenna goes up, folds over
the top and back down to the other side of the roof where it feeds
thru a high voltage vacuum variable capacitor back to ground.

Isn't this like the Q-Mac antenna sold in Australia?
http://www.qmac.com/mobile_antennas.html



---------------------------
| |
|_ |
| |( coax conx) = Capacitor
| -|_____________|
^ (roof)
^
(feed)

I am not as good an artist as some of you, but here is my graphic
The coax feeds with a gamma? and is tuned by a stepper motor tuning
the vacuum variable cap. Depending on which of the three models, it
operates from 1.5-10 MHz, 2.7-15.5 MHz, or 4.5-22 MHz at up to 150
watts.

The technical notes have more interesting details:
http://stealth.ae/plugins/custompage...cles.php?id=10

It seems to me a homebrew version could be made for base use such as
for those in restricted homes. Placed on the peak of the house, the
ground side could be a wire across the top of the peak and the loop
lift up during use so it would not attract too much attention to the
neighbors.

Since the roof of a house is fairly large, a more conventional random
length loop tuned with something like a LDG or SGC autotuner at the
feedpoint might be a good approach. Icom IC-AH4 could probably also work.



I wonder what the size and cost of the Vacuum variable is to make a
similar antenna??


size of a pickle jar
Several hundred dollars, depending on size, power handling, and source..

http://www.surplussales.com/VaccumVarCaps/VVC5.html
http://www.mgs4u.com/RF-Microwave/va...citors-500.htm

plus you'll need to cobble up a motor and drive system.

the $300 or so for a autotuner starts to look like a good deal.
http://www.hamradio.com/web/newcat/hrocat7.pdf




Buck
N4PGW

Jim Lux wrote:
Buck wrote:

This is an interesting antenna. It is a mobile stealth magnetic loop
antenna.

http://stealth.ae/plugins/custompage...categories=136

I watched the video and checked out the details. The antenna is
basically a loop with the roof of the car (the bottom of the luggage
rack) as the ground side of the loop, the antenna goes up, folds over
the top and back down to the other side of the roof where it feeds
thru a high voltage vacuum variable capacitor back to ground.



Actually, at 150W, it might not be using a vacuum variable, just a
suitable air variable with big spacing. Electrically, this is much the
same as the AEA isoloop or MFJ-1782 type antenna. The ARRL antenna book
has a whole chapter on them, generically.

On Fri, 21 Mar 2008 15:50:45 -0700, Jim Lux
wrote:

Buck wrote:
This is an interesting antenna. It is a mobile stealth magnetic loop
antenna.

http://stealth.ae/plugins/custompage...categories=136

Isn't this like the Q-Mac antenna sold in Australia?
http://www.qmac.com/mobile_antennas.html


They sure look the same.


Since the roof of a house is fairly large, a more conventional random
length loop tuned with something like a LDG or SGC autotuner at the
feedpoint might be a good approach. Icom IC-AH4 could probably also work.

That is certainly an idea I hadn't thought of. It sounds cheaper and
easier to build. The same concept could be used with raising and
lowering the antenna, but instead of a 'loop', it could be a wire.

Thought: let's assume one puts up a loop on the roof. The ground side
is a single wire running over the peak of the house roof, folds up at
90 degrees for 3-6 feet, back across the roof 6 foot above the peak
and back down to the hot lead of the autotuner. Would that not
circumvent the need for the capacitor at the far end? and would the
loop have to be trimmed to a particular size to operate the various
bands, or could the tuner actually operate with most any size loop.

Disclaimer: I know from experience that some random lengths might not
work on all bands, but some do appear to. I recognize that there
could be inefficiencies...




I wonder what the size and cost of the Vacuum variable is to make a
similar antenna??


size of a pickle jar
Several hundred dollars, depending on size, power handling, and source..

http://www.surplussales.com/VaccumVarCaps/VVC5.html
http://www.mgs4u.com/RF-Microwave/va...citors-500.htm

plus you'll need to cobble up a motor and drive system.

the $300 or so for a autotuner starts to look like a good deal.
http://www.hamradio.com/web/newcat/hrocat7.pdf



I tend to agree with you here




Buck
N4PGW

This is what I was fishing for. Maybe the antenna design isn't the
best, but the discussion is introducing variations of thinking that
could be useful.

Thank you
Buck
--
73 for now
Buck, N4PGW

www.lumpuckeroo.com

"Small - broadband - efficient: pick any two."

Buck wrote:
On Fri, 21 Mar 2008 15:50:45 -0700, Jim Lux
wrote:



Since the roof of a house is fairly large, a more conventional random
length loop tuned with something like a LDG or SGC autotuner at the
feedpoint might be a good approach. Icom IC-AH4 could probably also work.


That is certainly an idea I hadn't thought of. It sounds cheaper and
easier to build. The same concept could be used with raising and
lowering the antenna, but instead of a 'loop', it could be a wire.

Thought: let's assume one puts up a loop on the roof. The ground side
is a single wire running over the peak of the house roof, folds up at
90 degrees for 3-6 feet, back across the roof 6 foot above the peak
and back down to the hot lead of the autotuner. Would that not
circumvent the need for the capacitor at the far end? and would the
loop have to be trimmed to a particular size to operate the various
bands, or could the tuner actually operate with most any size loop.

Probably would work ok. The other approach would be a more conventional
dipole looking thing, with the tuner in the middle. To a first order,
they'll work the same. A long skinny (much wavelength wide) loop
looks a lot like a folded dipole. So, how well it works will probably be
determined more by what impedance happens to appear at the feedline, and
how well the tuner matches it. If the tuner's at the end (either of the
skinny loop or a long random wire) then the impedance tends to be high
(close to resonance). If it's in the middle, it's low.

In your situation, I might be tempted to try hanging a vertical wire
down the side of the house, hooked to one side of the tuner, and the
other horizontalish wire 6 ft off the roof. RFI/EMI might be an issue..

You could also do something like run a couple wires on the roof at right
angles to the ridge line, hook those to one terminal of the tuner, then
hook your wire running along the ridge to the other terminal.


I've fooled around a lot with various "throw the wire on the roof"
situations (out of laziness, more than anything), and, in general, it
seems to work better (no quantititive results) when it looks more like a
fan dipole (feedpoint in the middle-ish) than a loop (feedpoint on the
edge).

On Mon, 24 Mar 2008 08:49:49 -0700, Jim Lux
wrote:


I've fooled around a lot with various "throw the wire on the roof"
situations (out of laziness, more than anything), and, in general, it
seems to work better (no quantititive results) when it looks more like a
fan dipole (feedpoint in the middle-ish) than a loop (feedpoint on the
edge).

My original post was to generate topics on the antenna design and/or
equivalent or better stealth alternatives. I don't really need to
drop the wire, but that too is a good idea.

As for the 'fan dipole' you like, I have been using the horizontal
version throughout my 30 years of ham radio. I learned about the
two-wire version as a novice when I was first licensed. 80 meters cut
for the Novice frequencies also doubled as a 10 meter dipole. 40
meters cut at the lower end of the Novice frequencies doubled as a 15
meter antenna.

As a General Class operator, I discovered I not only had to add an
element for 20 meters, but the 75 meter dipole didn't work on 10 and
the 40 meter dipole didn't work on 15 meters ??? Then I learned about
where the harmonics fell.

By the time I became Advanced class, we had three new WARC bands.
Along with 75, 80, 40, 30, 20, 17, 15, 12, & 10 meters comes 9 wires
or an antenna tuner. I never did like antenna tuners. I always
wanted one antenna to do it all. It is amazing how quickly I fell in
love with my tuner, though, when I hung a 130 foot dipole about 100
feet high center fed with 300 ohm twin lead. I learned to tune the
antenna almost as quickly as I could throw an antenna switch a couple
of times.

Now that auto-tuners are about as inexpensive as the manual tuners, I
am awfully tempted to wander over to the SGC shelf at one of the ham
stores and try it out.

I am moving to a new house soon. We will be restricted in antennas
only to the extent that we don't make the yard look too ugly. It
isn't terribly well suited for many antennas, but I do hope to put up
at least one decent fan-dipole, more accurately parallel dipole. I
don't think I can fit a full-wave loop, due to lack of supports, but
it looks like I'll have a place outside my window where a row of trees
can support a 60 foot or so high dipole.... I still have a 100 foot
roll of 300 or 450 ohm window line


I guess I digressed.
73 for now
Buck
N4PGW

--
73 for now
Buck, N4PGW

www.lumpuckeroo.com

"Small - broadband - efficient: pick any two."

Buck wrote:


Now that auto-tuners are about as inexpensive as the manual tuners, I
am awfully tempted to wander over to the SGC shelf at one of the ham
stores and try it out.

I am moving to a new house soon. We will be restricted in antennas
only to the extent that we don't make the yard look too ugly. It
isn't terribly well suited for many antennas, but I do hope to put up
at least one decent fan-dipole, more accurately parallel dipole. I
don't think I can fit a full-wave loop, due to lack of supports, but
it looks like I'll have a place outside my window where a row of trees
can support a 60 foot or so high dipole.... I still have a 100 foot
roll of 300 or 450 ohm window line


The advantage of the tuner with a "fan" or "parallel wire" dipole is
that if you put it at the feedpoint, the incredibly tedious pruning
process is eliminated. You just have to get a couple wires "about
right", and the tuner does the rest (who cares if the resonance is at
13.5 MHz.. that's not a huge change). And, in the "thrown on the roof"
sort of scenario, as the roof grunge and moisture changes or the trees
grow up or get leaves on them, changing the native Z of the antenna, the
tuner automatically compensates.

It's a coax feedline, so it's easy to route. The matching is at the
antenna, so the loss in the feedline is less of an issue.

In this sort of thing it *is* useful to have some way to see if the
antenna has fallen apart or shorted out or is covered with wet leaves.
The tuner will happily tune almost anything, but you have no real way to
know if it's actually radiating. I've used a couple techniques.. a
monitor receiver with a whip antenna is one; looking at NCDXF beacons or
WWV is another.

On Mar 25, 11:32 am, Jim Lux wrote:
Buck wrote:

Now that auto-tuners are about as inexpensive as the manual tuners, I
am awfully tempted to wander over to the SGC shelf at one of the ham
stores and try it out.

I am moving to a new house soon. We will be restricted in antennas
only to the extent that we don't make the yard look too ugly. It
isn't terribly well suited for many antennas, but I do hope to put up
at least one decent fan-dipole, more accurately parallel dipole. I
don't think I can fit a full-wave loop, due to lack of supports, but
it looks like I'll have a place outside my window where a row of trees
can support a 60 foot or so high dipole.... I still have a 100 foot
roll of 300 or 450 ohm window line

The advantage of the tuner with a "fan" or "parallel wire" dipole is
that if you put it at the feedpoint, the incredibly tedious pruning
process is eliminated. You just have to get a couple wires "about
right", and the tuner does the rest (who cares if the resonance is at
13.5 MHz.. that's not a huge change). And, in the "thrown on the roof"
sort of scenario, as the roof grunge and moisture changes or the trees
grow up or get leaves on them, changing the native Z of the antenna, the
tuner automatically compensates.

It's a coax feedline, so it's easy to route. The matching is at the
antenna, so the loss in the feedline is less of an issue.

In this sort of thing it *is* useful to have some way to see if the
antenna has fallen apart or shorted out or is covered with wet leaves.
The tuner will happily tune almost anything, but you have no real way to
know if it's actually radiating. I've used a couple techniques.. a
monitor receiver with a whip antenna is one; looking at NCDXF beacons or
WWV is another.

You can put your radio in a hand carry wire shopping basket and
dispense with the transmission line. Wireless shopping no less.

On Tue, 25 Mar 2008 09:32:51 -0700, Jim Lux
wrote:

Buck wrote:


Now that auto-tuners are about as inexpensive as the manual tuners, I
am awfully tempted to wander over to the SGC shelf at one of the ham
stores and try it out.

I am moving to a new house soon. We will be restricted in antennas
only to the extent that we don't make the yard look too ugly. It
isn't terribly well suited for many antennas, but I do hope to put up
at least one decent fan-dipole, more accurately parallel dipole. I
don't think I can fit a full-wave loop, due to lack of supports, but
it looks like I'll have a place outside my window where a row of trees
can support a 60 foot or so high dipole.... I still have a 100 foot
roll of 300 or 450 ohm window line


The advantage of the tuner with a "fan" or "parallel wire" dipole is
that if you put it at the feedpoint, the incredibly tedious pruning
process is eliminated. You just have to get a couple wires "about
right", and the tuner does the rest (who cares if the resonance is at
13.5 MHz.. that's not a huge change). And, in the "thrown on the roof"
sort of scenario, as the roof grunge and moisture changes or the trees
grow up or get leaves on them, changing the native Z of the antenna, the
tuner automatically compensates.

It's a coax feedline, so it's easy to route. The matching is at the
antenna, so the loss in the feedline is less of an issue.

In this sort of thing it *is* useful to have some way to see if the
antenna has fallen apart or shorted out or is covered with wet leaves.
The tuner will happily tune almost anything, but you have no real way to
know if it's actually radiating.

Actually, a field strength meter in the radio room works wonders. If
it quits deflecting and you are pumping full power... oops.



I've used a couple techniques.. a
monitor receiver with a whip antenna is one; looking at NCDXF beacons or
WWV is another.

--
73 for now
Buck, N4PGW

www.lumpuckeroo.com

"Small - broadband - efficient: pick any two."