Bricks effect in dipole resonance? Help!
 

I'd guess that a brick wall would lower the resonance somewhat (due to
the brick's dielectric constant), but not introduce a significant amount
of loss. But that's a guess.

What I'd do, of course, is model the antenna with EZNEC (the free demo
program would be adequate). That would tell me where the antenna would
be resonant without the brick wall. Any major deviation from that could
be attributed to the brick wall. Of course, you'd have to include your
coil in the model, including its loss if you want to determine the
antenna loss, and the feedline to account for its impedance
transformation and consequent effect on resonance. To determine the
antenna loss I'd look at the bandwidth with EZNEC and compare it to the
measured bandwidth -- if the real antenna is considerably broader,
there's extra loss from somewhere that's not in the model.

If you don't think it's worth the effort to model, you might just try
doubling the distance from the wall and remeasuring. I'm not sure that
would be enough to really tell, but if the resonance changes noticeably,
then the bricks are having an effect. And if the bandwidth decreases
appreciably, then the bricks are contributing loss.

Roy Lewallen, W7EL

Ken Bessler wrote:
What effect would a brick wall have on a HF antenna?
I'm talking 1-2" distance from the bricks, folks. The
wire used is 22 ga stranded insulated speaker wire.
Now no lectures about power - I run QRP. :-) The
station is well grounded to a copper baseboard heater
pipe and all components have short runs (12" or less)
of 1/4" braid line going to a common point. From there
it's 1/4" braid (26" long) to the pipe. I can touch any
part of the system while transmitting and see no change
in SWR.

I cut an inverted V 66'7" per leg 133'2" overall. It's fed
with 12' of rg8x coax with a 6 turn 2-5/8" dia coil at the
feedpoint. The apex of the antenna is roughly 25' up. The
ends of the antenna are at chest height. The antenna is bent
around the corner of my building 90 degrees (I'm in a
corner unit). The last 8' of each leg bends again 90 degrees.

The antenna resonates as follows:
============
2.130 2:1 swr
2.715 1:1 swr (flat)
3.620 2:1 swr
============
8.970 2:1 swr
9.210 1.8:1 swr
9.390 2:1 swr
============
12.230 2:1 swr
13.040 1.9:1 swr
13.850 2:1 swr
============
15.930 2:1 swr
============
20.970 2:1 swr
22.745 1.2:1 swr
24.520 2:1 swr
============

If I figure 468/133.1666666667 I get 3.514.393 mhz
for resonance How come the antenna is so far off?
Could it be the height? Not that I'm complaining - I
like the low resonance and my Z11 autotuner will take
this antenna down to 1.8 mhz real easy. Although at
that freq I expect the antenna will radiate a NVIS
signal due to low HAAT.

Any ideas???

72's de Ken KG0WX

Ken:

I've not run HF measurements on bricks, but I can tell you that a brick wall
is extremely lossy at 1 GHz and above, by measurement. Their water
(dielectric constant = 80) content is relatively high unless recently baked
and sealed, and depending on the particular clay they were fired from, they
are somewhat conductive. So, placing your antenna close is like putting it
next to a high dielectric RF absorber. Not a good choice.

--
Crazy George
Remove NO and SPAM from return address
"Ken Bessler" wrote in message
...
What effect would a brick wall have on a HF antenna?
I'm talking 1-2" distance from the bricks, folks. The
wire used is 22 ga stranded insulated speaker wire.
Now no lectures about power - I run QRP. :-) The
station is well grounded to a copper baseboard heater
pipe and all components have short runs (12" or less)
of 1/4" braid line going to a common point. From there
it's 1/4" braid (26" long) to the pipe. I can touch any
part of the system while transmitting and see no change
in SWR.

I cut an inverted V 66'7" per leg 133'2" overall. It's fed
with 12' of rg8x coax with a 6 turn 2-5/8" dia coil at the
feedpoint. The apex of the antenna is roughly 25' up. The
ends of the antenna are at chest height. The antenna is bent
around the corner of my building 90 degrees (I'm in a
corner unit). The last 8' of each leg bends again 90 degrees.

The antenna resonates as follows:
============
2.130 2:1 swr
2.715 1:1 swr (flat)
3.620 2:1 swr
============
8.970 2:1 swr
9.210 1.8:1 swr
9.390 2:1 swr
============
12.230 2:1 swr
13.040 1.9:1 swr
13.850 2:1 swr
============
15.930 2:1 swr
============
20.970 2:1 swr
22.745 1.2:1 swr
24.520 2:1 swr
============

If I figure 468/133.1666666667 I get 3.514.393 mhz
for resonance How come the antenna is so far off?
Could it be the height? Not that I'm complaining - I
like the low resonance and my Z11 autotuner will take
this antenna down to 1.8 mhz real easy. Although at
that freq I expect the antenna will radiate a NVIS
signal due to low HAAT.

Any ideas???

72's de Ken KG0WX

Hm, woops. My guess that they wouldn't be lossy was based on the
assumption that they don't contain much water. In light of what George
has said, I retract my guess. Putting your antenna close to anything
containing water is bad news from a loss standpoint, as George says.

Roy Lewallen, W7EL

Crazy George wrote:
Ken:

I've not run HF measurements on bricks, but I can tell you that a brick wall
is extremely lossy at 1 GHz and above, by measurement. Their water
(dielectric constant = 80) content is relatively high unless recently baked
and sealed, and depending on the particular clay they were fired from, they
are somewhat conductive. So, placing your antenna close is like putting it
next to a high dielectric RF absorber. Not a good choice.

--
Crazy George

"Roy Lewallen" wrote in message
...
Hm, woops. My guess that they wouldn't be lossy was based on the
assumption that they don't contain much water. In light of what George
has said, I retract my guess. Putting your antenna close to anything
containing water is bad news from a loss standpoint, as George says.

Roy Lewallen, W7EL

Crazy George wrote:
Ken:

I've not run HF measurements on bricks, but I can tell you that a brick
wall
is extremely lossy at 1 GHz and above, by measurement. Their water
(dielectric constant = 80) content is relatively high unless recently
baked
and sealed, and depending on the particular clay they were fired from,
they
are somewhat conductive. So, placing your antenna close is like putting
it
next to a high dielectric RF absorber. Not a good choice.

--
Crazy George


Well, that expains the wide bandwidth between the 2:1 swr points
(1490 khz)......

Still, I am VERY limited as to my options - I rent here - so this
antenna is it or else I go with a Superantennas MP1 stuck out
the window. While the MP1 is a superb performer for it's size,
I don't think it could do any better than what I've got. Then there
is the fact that I'd have to have the window open to make room
for the MP1 - not a good idea in january in colorado.

Now I haven't had a chance yet to see how good the antenna
works on 160 aside from SWR/bandwith weasuring but tonight
on 80 the antenna seemed to hear pretty well - I'm hearing a lot
of 1, 2, 3 & 6 calls all over the band. If the bricks are absorbing
signals they're not doing too much damage. I'm hearing WWVH
on 5 mhz ok, also and I'm hearing a lot of DX AM broadcast
band stations - It's amazing how crowded that band is at 4 am!

Now I do have the option of taking 1 leg and running it away
from the bricks to a nearby tree but the other has to stay where
it is - that leg runs most of it's length about 5' away from the
wall due to the shape of the building. Problem is that the tree
is on the neighbor's property so that's probably out.

Anyways, the performance seems nominal and I don't have
many choices so I guess I'll have to be satisfied with what
I've got.

72's de Ken KG0WX

What kind of SWR did you get on frequencies not in your table? I would
expect to see some 10:1 and 30:1 type numbers.

Tam/WB2TT
"Ken Bessler" wrote in message
...

The antenna resonates as follows:
============
2.130 2:1 swr
2.715 1:1 swr (flat)
3.620 2:1 swr
============
8.970 2:1 swr
9.210 1.8:1 swr
9.390 2:1 swr
============
12.230 2:1 swr
13.040 1.9:1 swr
13.850 2:1 swr
============
15.930 2:1 swr
============
20.970 2:1 swr
22.745 1.2:1 swr
24.520 2:1 swr
============

If I figure 468/133.1666666667 I get 3.514.393 mhz
for resonance How come the antenna is so far off?
Could it be the height? Not that I'm complaining - I
like the low resonance and my Z11 autotuner will take
this antenna down to 1.8 mhz real easy. Although at
that freq I expect the antenna will radiate a NVIS
signal due to low HAAT.

Any ideas???

72's de Ken KG0WX

Ken:

With a short attention span, I'm not sure I understand the tree option you
mention, but if you can run one leg away from the wall, at near a right
angle, then put 2,3, 4 or more wires on the wall and call that your
counterpoise. Feed the wire against that. Picture a ground plane turned on
its side with the driven element running horizontal. That will get most of
the field away from the bricks.

--
Crazy George
Remove NO and SPAM from return address
"Ken Bessler" wrote in message
...

"Roy Lewallen" wrote in message
...
Hm, woops. My guess that they wouldn't be lossy was based on the
assumption that they don't contain much water. In light of what George
has said, I retract my guess. Putting your antenna close to anything
containing water is bad news from a loss standpoint, as George says.

Roy Lewallen, W7EL

Crazy George wrote:
Ken:

I've not run HF measurements on bricks, but I can tell you that a
brick
wall
is extremely lossy at 1 GHz and above, by measurement. Their water
(dielectric constant = 80) content is relatively high unless recently
baked
and sealed, and depending on the particular clay they were fired from,
they
are somewhat conductive. So, placing your antenna close is like
putting
it
next to a high dielectric RF absorber. Not a good choice.

--
Crazy George


Well, that expains the wide bandwidth between the 2:1 swr points
(1490 khz)......

Still, I am VERY limited as to my options - I rent here - so this
antenna is it or else I go with a Superantennas MP1 stuck out
the window. While the MP1 is a superb performer for it's size,
I don't think it could do any better than what I've got. Then there
is the fact that I'd have to have the window open to make room
for the MP1 - not a good idea in january in colorado.

Now I haven't had a chance yet to see how good the antenna
works on 160 aside from SWR/bandwith weasuring but tonight
on 80 the antenna seemed to hear pretty well - I'm hearing a lot
of 1, 2, 3 & 6 calls all over the band. If the bricks are absorbing
signals they're not doing too much damage. I'm hearing WWVH
on 5 mhz ok, also and I'm hearing a lot of DX AM broadcast
band stations - It's amazing how crowded that band is at 4 am!

Now I do have the option of taking 1 leg and running it away
from the bricks to a nearby tree but the other has to stay where
it is - that leg runs most of it's length about 5' away from the
wall due to the shape of the building. Problem is that the tree
is on the neighbor's property so that's probably out.

Anyways, the performance seems nominal and I don't have
many choices so I guess I'll have to be satisfied with what
I've got.

72's de Ken KG0WX

Very interesting.

There's a little data on the RF properties of such things, but it's
pretty sparse and limited in scope.

Seems to me that the thing to do would be wind a big air core coil, and
measure its inductance and Q. Compare that to the inductance and Q with
the material under test inserted into the center of the coil. Given
similar size samples, that should at least give an indication of
comparitive loss and dielectric constant of the materials. Quantitative
results could probably be had with a bit of thought and calculation.

Or maybe just stick a plate on each side of the sample to make a
capacitor, and measure the capacitance and Q. One method would probably
work better for some materials and the other for others.

There's no doubt that water is really bad stuff at HF.

Roy Lewallen, W7EL

Crazy George wrote:
Roy:

I get the impression that building materials survive by being full of water.
I had the chance to measure cinder blocks which had been dehydrated ( poor
RF) and watch them gather moisture over a year or so (really, really bad
after a year). And that was inside an air conditioned lab. I suspect the
process would have taken only days outside in our 90%+ humidity.

Sheetrock = bad, moisture
Clay tile = better, glazed to prevent moisture penetration
wood = bad, moisture, even after years in 100 degree attics, interestingly
enough.
Styrofoam with a thin polymer coating (various proprietary wall systems), =
pretty good, unless the interface wicks moisture in because the wrong glue
is used.
ad nauseum
--
Crazy George
Remove NO and SPAM from return address

"Tarmo Tammaru" wrote in message
...
What kind of SWR did you get on frequencies not in your table? I would
expect to see some 10:1 and 30:1 type numbers.

My only current way to measure SWR is with a bridge and a low
power transmitter. The limit of this device is 5:1. At times, the SWR
inicated 4-4.5:1 but never higher......

I've since added 10' to each leg and here are the latest numbers:

2:1 swr bandwidth and resonance points:
1.990 - 2.640, 1:1 @ 2.330 mhz (was 2.715)
minor dip to 2:1 @ 8.070 mhz
11.680 - 14.390, 1.2:1 @ 13.610 mhz
21.630 - 25.250, 1.2:1 @ 23.640 mhz

Total antenna length 153'2" (76'7" per leg)

73's de Ken KG0WX

Ken Bessler wrote:

What effect would a brick wall have on a HF antenna?
I'm talking 1-2" distance from the bricks, folks. The
wire used is 22 ga stranded insulated speaker wire.
Now no lectures about power - I run QRP. :-) The
station is well grounded to a copper baseboard heater
pipe and all components have short runs (12" or less)
of 1/4" braid line going to a common point. From there
it's 1/4" braid (26" long) to the pipe. I can touch any
part of the system while transmitting and see no change
in SWR.

I cut an inverted V 66'7" per leg 133'2" overall. It's fed
with 12' of rg8x coax with a 6 turn 2-5/8" dia coil at the
feedpoint. The apex of the antenna is roughly 25' up. The
ends of the antenna are at chest height. The antenna is bent
around the corner of my building 90 degrees (I'm in a
corner unit). The last 8' of each leg bends again 90 degrees.

The antenna resonates as follows:
============
2.130 2:1 swr
2.715 1:1 swr (flat)
3.620 2:1 swr
============
8.970 2:1 swr
9.210 1.8:1 swr
9.390 2:1 swr
============
12.230 2:1 swr
13.040 1.9:1 swr
13.850 2:1 swr
============
15.930 2:1 swr
============
20.970 2:1 swr
22.745 1.2:1 swr
24.520 2:1 swr
============

If I figure 468/133.1666666667 I get 3.514.393 mhz
for resonance How come the antenna is so far off?
Could it be the height? Not that I'm complaining - I
like the low resonance and my Z11 autotuner will take
this antenna down to 1.8 mhz real easy. Although at
that freq I expect the antenna will radiate a NVIS
signal due to low HAAT.

Any ideas???

72's de Ken KG0WX

I used to run a 40M dipole which lay along a rock wall 3 feet
off the ground. I cut it for the center of the band, laid it
out (held in place with a few bricks), and worked the world!

It was just a few bricks short of a full load! Seriously, though,
it worked well.
--
--------------------------------------
Diagnosed Type II Diabetes March 5 2001
Beating it with diet and exercise!
297/215/210 (to be revised lower)
58"/43"(!)/44" (already lower too!)
--------------------------------------
Visit my HomePage at http://members.shaw.ca/finkirv/
Visit my very special website at http://members.shaw.ca/finkirv4/
Visit my CFSRS/CFIOG ONLINE OLDTIMERS website at http://members.shaw.ca/finkirv5/
--------------------
Irv Finkleman,
Grampa/Ex-Navy/Old Fart/Ham Radio VE6BP
Calgary, Alberta, Canada

"Ken Bessler" wrote in message
...

I've run the antenna through EZNEC. I didn't include the feedline
coil because I did'nt know how to model it. I'm still learning how
to use the program.

EZNEC not only shows a differant resonance point (3.2:1 @ 9.5 mhz
with Z=19.81 - j 24.27 ohms) but shows a tighter bandwidth.

Update: I'm still learning how to use EZNEC but I changed the soil
type to MiniNEC and now show three swr dips, just like the real
antenna. The only differance is EZNEC shows them about 1 mhz
higher and with about 1/4 the bandwidth that I see.....

Must be the bricks.... :-)

Ken KG0WX