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Help! Apartment Antenna Question!



 
 
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  #1  
Old January 24th 04, 05:43 PM
Jeff
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Default Help! Apartment Antenna Question!

Hello.

I live in a third floor apartment without a balcony. As such, finding a
way to erect an antenna has been a challenge. My home station is a 2M
mobile transceiver (Yaesu FT-2800M) which I've been attempting to find
an optimal antenna arrangement without satisfaction.

I originally constructed a J-pole from 450-ohm ladder line that I had
hanging in our bedroom corner (the corner where it displayed the best
SWR), but the walls of this apartment apparently have alot of metal in
them that apparently affect my signal from it. I've attempted hanging it
in various places around our place and I've not gotten good reports.
Also, I exceed RF safety when I work simplex on 65W with this. My power
supply also lets me know this by buzzing during transmission. So.. I had
to find a different method.

We've got a wall air conditioner sticking out the side of our building
with a metal box covering it. I can place a mag mount on and attach a
mobile antenna to with satisfactory results. However, a small base
magmount like an MFJ dual band (a 3" diameter mag base) seems to work
quite well with a flat SWR, but I'd like to attach a more efficient
antenna to it.

I've got a 5" base Comet mag mount that I've attached both a Diamond
NR2C (2M 4.1dB center loaded mobile antenna) and a Diamond SG7900 (a
huge dualband antenna giving 5.0dB on 2M). Upon testing them on my AC
box placement, the SWRs exceed 2.0:1! Nothign I can do seems to improve
the SWR with these more efficient antennas.

My speculation is that the larger mag-mount base needs a larger ground
plane than the AC box can provide, while the smaller MFJ magmount is
sufficiently matched.

Does anyone have a suggestion on why this is occuring? Better yet, is
there a way I can possibly put a larger mobile antenna on this AC box
and get satisfactory SWR?

Thanks for any suggestions!

73s
Jeff
DE KB1HOL/AG
--
--
J. Rogers KB1HOL
KB1HOLatARRLdotNET
  #2  
Old January 24th 04, 07:09 PM
Cecil Moore
Guest
 
Posts: n/a
Default

Jeff wrote:
I've got a 5" base Comet mag mount that I've attached both a Diamond
NR2C (2M 4.1dB center loaded mobile antenna) and a Diamond SG7900 (a
huge dualband antenna giving 5.0dB on 2M). Upon testing them on my AC
box placement, the SWRs exceed 2.0:1! Nothign I can do seems to improve
the SWR with these more efficient antennas.


What does a plot of SWR Vs frequency look like across the band?
--
73, Cecil http://www.qsl.net/w5dxp



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  #3  
Old January 24th 04, 11:51 PM
Gary S.
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Posts: n/a
Default

On Sat, 24 Jan 2004 12:43:39 -0500, Jeff
wrote:

Hello.

I live in a third floor apartment without a balcony. As such, finding a
way to erect an antenna has been a challenge. My home station is a 2M
mobile transceiver (Yaesu FT-2800M) which I've been attempting to find
an optimal antenna arrangement without satisfaction.

I originally constructed a J-pole from 450-ohm ladder line that I had

We've got a wall air conditioner sticking out the side of our building


My speculation is that the larger mag-mount base needs a larger ground
plane than the AC box can provide, while the smaller MFJ magmount is
sufficiently matched.

Does anyone have a suggestion on why this is occuring? Better yet, is
there a way I can possibly put a larger mobile antenna on this AC box
and get satisfactory SWR?

For reference, a good ground plane should be a least a radius of 14
wavelength around the base of the antenna. Two strips of metal at
right angles works reasonably, and the plane need not be solid, aas
long as the holes are not too large in relation to the wavelength.

Theoretically speaking, a ground plane for 2M should be a radius of
19.25 inches.

Practically speaking, I have had good results with a magmount 2M on a
standard cookie sheet, inside the house (single story wood
construction).

There is a book called "Stealth Antennas" which may help you.

Also keep in mind that a vertical antenna can hang down from the feed
point, as well as going up from there.

Happy trails,
Gary (net.yogi.bear)
------------------------------------------------
at the 51st percentile of ursine intelligence

Gary D. Schwartz, Needham, MA, USA
Please reply to: garyDOTschwartzATpoboxDOTcom
  #4  
Old January 25th 04, 02:24 AM
Red
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Posts: n/a
Default

Jeff wrote in message ...
Hello.

I live in a third floor apartment without a balcony. As such, finding a
way to erect an antenna has been a challenge. My home station is a 2M
mobile transceiver (Yaesu FT-2800M) which I've been attempting to find
an optimal antenna arrangement without satisfaction.

I originally constructed a J-pole from 450-ohm ladder line that I had
hanging in our bedroom corner (the corner where it displayed the best
SWR), but the walls of this apartment apparently have alot of metal in
them that apparently affect my signal from it. I've attempted hanging it
in various places around our place and I've not gotten good reports.
Also, I exceed RF safety when I work simplex on 65W with this. My power
supply also lets me know this by buzzing during transmission. So.. I had
to find a different method.

We've got a wall air conditioner sticking out the side of our building
with a metal box covering it. I can place a mag mount on and attach a
mobile antenna to with satisfactory results. However, a small base
magmount like an MFJ dual band (a 3" diameter mag base) seems to work
quite well with a flat SWR, but I'd like to attach a more efficient
antenna to it.

I've got a 5" base Comet mag mount that I've attached both a Diamond
NR2C (2M 4.1dB center loaded mobile antenna) and a Diamond SG7900 (a
huge dualband antenna giving 5.0dB on 2M). Upon testing them on my AC
box placement, the SWRs exceed 2.0:1! Nothign I can do seems to improve
the SWR with these more efficient antennas.

My speculation is that the larger mag-mount base needs a larger ground
plane than the AC box can provide, while the smaller MFJ magmount is
sufficiently matched.

Does anyone have a suggestion on why this is occuring? Better yet, is
there a way I can possibly put a larger mobile antenna on this AC box
and get satisfactory SWR?

Thanks for any suggestions!

73s
Jeff
DE KB1HOL/AG
--


In apartments wanting to work 2 meters, I've used a mobile magmount on
anything made of metal, such as a metal panel for the central air and
heat, a refrigerator, a stove(when I wasn't cooking of course), a
metal cabinet, and have had good results with both a 1/4wave and a
5/8wave magmount.

It worked well for repeaters and simplex. I'm currently running a
1/4wave vertical on top of a metal cabinet and with just a few watts
on a handheld, I can put out a good signal for miles simplex.

Hope this helps.

Red
  #5  
Old January 25th 04, 03:34 AM
'Doc
Guest
 
Posts: n/a
Default



Jeff,
Try using the AC as a mounting point for a horizontal
bar running out far enough to mount an antenna on, such
as some type of 'base'(?) antenna.
All antennas have to be tuned and/or impedance matched.
Any antenna near obstructions will have undesirable
characteristics compared to the same antenna in the 'clear'.
I think what you're seeing is fairly 'normal' for your
situation. Good luck...
'Doc
  #6  
Old January 26th 04, 04:20 AM
Jeff
Guest
 
Posts: n/a
Default

In article ,
Cecil Moore wrote:

Jeff wrote:
I've got a 5" base Comet mag mount that I've attached both a Diamond
NR2C (2M 4.1dB center loaded mobile antenna) and a Diamond SG7900 (a
huge dualband antenna giving 5.0dB on 2M). Upon testing them on my AC
box placement, the SWRs exceed 2.0:1! Nothign I can do seems to improve
the SWR with these more efficient antennas.


What does a plot of SWR Vs frequency look like across the band?


With the smaller (3") MFJ magmount, its as follows:

144Mhz 1.8
145Mhz 1.7
146Mhz 1.5
147Mhz 1.1
148Mhz 1.1

Clearly the antenna is not optimally tuned for the 2M amateur bandwidth,
and I could probably get the SWR bell curve down to 146 by lengthening
the antenna. However, 99% of my use is on the high end of the band so
it works just fine. Its a small antenna and I want something with more
gain, thus I've not fiddled with it to improve the SWR.

With the larger 5" mag-mount on the SG7900:

144Mhz 3.0
145Mhz 3.0
146Mhz 2.5
147Mhz 2.5
148Mhz 3.0

SWR profile is similar with both this antenna and the 2M monobander
Diamond NR2C.

73s
Jeff
--
--
J. Rogers KB1HOL
KB1HOLatARRLdotNET
  #7  
Old January 26th 04, 04:26 AM
Jeff
Guest
 
Posts: n/a
Default

In article ,
Gary S. Idontwantspam@net wrote:

For reference, a good ground plane should be a least a radius of 14
wavelength around the base of the antenna. Two strips of metal at
right angles works reasonably, and the plane need not be solid, aas
long as the holes are not too large in relation to the wavelength.

Theoretically speaking, a ground plane for 2M should be a radius of
19.25 inches.


The AC box is rectangular and I've got about 19" on two sides, but less
than that in the other two.

My understanding is this ground plane requirement for a magmount doesn't
change much with each magmount? It could be the larger 5" magmount
requires more metal?

Practically speaking, I have had good results with a magmount 2M on a
standard cookie sheet, inside the house (single story wood
construction).


I could do that, but I want to keep the antenna as far away from people
as possible since I'm on 65W simplex frequently.

Long story short, I'm trying to figure out why the metal AC box
functions just fine for a smaller magmount, but not the larger one.

There is a book called "Stealth Antennas" which may help you.


I'll keep my eye out, though most stealth antennas seem to require
access to space than I can provide.

Also keep in mind that a vertical antenna can hang down from the feed
point, as well as going up from there.


Absolutely, though I don't think our neighbor below us would appreciate
the whip hanging in front of his window.

73s
Jeff
--
--
J. Rogers KB1HOL
KB1HOLatARRLdotNET
  #8  
Old January 26th 04, 04:30 AM
Jeff
Guest
 
Posts: n/a
Default

In article , 'Doc wrote:

Jeff,
Try using the AC as a mounting point for a horizontal
bar running out far enough to mount an antenna on, such
as some type of 'base'(?) antenna.


I've tried to figure out a way to do exactly that. I'd love to get a
"stand off" far enough away so I could put on a simple quarter wave
ground plane, but it'd take a more mechanically clever person than I to
engineer such a thing off this AC box. Not only to do that, but to do
it so the maintainance guys here don't get upset with a modification to
the apartment's property.

All antennas have to be tuned and/or impedance matched.
Any antenna near obstructions will have undesirable
characteristics compared to the same antenna in the 'clear'.
I think what you're seeing is fairly 'normal' for your
situation. Good luck...



But the difference between the smaller and larger magmounts is what
puzzles me. Shouldn't both mounts give similar SWR readings?
--
--
J. Rogers KB1HOL
KB1HOLatARRLdotNET
  #9  
Old January 26th 04, 04:48 AM
Dave Platt
Guest
 
Posts: n/a
Default

In article ,
Jeff wrote:

But the difference between the smaller and larger magmounts is what
puzzles me. Shouldn't both mounts give similar SWR readings?


It would be worth checking to see whether the same antenna, on the
larger mag-mount, on a larger ground plane gives better results.
It's possible that it won't.

Most mag-mount antennas that I have seen, are designed to establish a
capacitive coupling with the metal ground plane, so that it'll
actually serve as an RF ground. The shield of the coax cable is
connected to a portion of the mag-mount base, and this is then
capacitively coupled to the car body (or whatever) in one way or
another. Some mag-mounts seem to depend on the magnet itself being a
good enough conductor to provide the coupling. Others place some sort
of thin, conductive metal foil across the bottom of the magnet
(sometimes with a protective plastic covering) and then connect this
foil to the coax braid somewhere inside the base.

It's possible that the large mag-mount base you're using doesn't have
enough coupling capacitance. Possibly, it was designed for use on an
HF antenna base, and isn't _intended_ to provide capacitive ground
coupling. At HF frequencies, capacitive coupling doesn't work very
well for mag-mounts, and the bases are usually designed to be grounded
to the car body via a wire or braid.

If your big mag-mount doesn't couple the ground to the air conditioner
base well enough, you'd probably end up with an ineffective ground and
a high SWR, and possibly one which changes as you move the coax
around. The outer portion of the coax braid would be working as a
counterpoise... and how well it works would depend on its position,
whether it was coiled up, how close it came to other metal objects,
etc.

If you want to be independent of the ground plane provided by your air
conditioner, you might want to investigate an end-fed half-wave
antenna. You can get mag-mountable commercial half-wave antennas
which use a matching coil in the base, and you can make (or perhaps
buy) a J-pole made out of something like thin brass or aluminum rod.
If you have an outdoor balcony with one of those large sliding glass
doors, I'll bet you could fabricate an on-the-glass J-pole, or a
center-fed vertical, out of adhesive-backed copper or aluminum tape
stuck to the glass.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
  #10  
Old January 26th 04, 01:05 PM
Richard Harrison
Guest
 
Posts: n/a
Default

Dave Platt wrote:
"Others place some sort of thin, conductive metal foil across the bottom
of the magnet."

Skin effect confines r-f to conductor surfaces. It is difficult to get
common-mode current through spaces as tiny as glue and paint thicknesses
on either side of the metal foil at the bottom of a mag-mount.. Polarity
of propagation at any instant is the same on all surfaces involved. The
little electrons avoid each other as much as possible on all surfaces.
Also, any current generates counter-emf which in a combination of such
surfaces tends to strongly oppose current, just as flowing into the
depths of aluminum or copper is opposed at r-f.

Dave also wrote:
"If you want to be independent of the ground plane provided by your air
conditioner, you might want to investigate an end-fed half-wave
antenna."

An end-fed antenna, regardless of its length, requires a ground or
something else to provide the second connection for the antenna.

A balanced antenna such as a dipole or loop is the only system
independent of the ground plane.

Best regards, Richard Harrison, KB5WZI

 




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