Open Loop aka Dipole bent into a square
 

I'm pressed for space outdoors so right now, I'm working with attic
antennas and need some help.

I was able to put a 60 foot loop of wire into my attic and am not
having much luck getting it to load up. I configured the loop as an
open loop, aka a dipole bent into a rectangle which is open at one
corner and fed with 300 ohm twinlead at the other. I'm using an MFJ
911H 4:1 and 1:1 balun to tranform the twinlead to coax and a homebrew
antenna tuner.

Using an SWR analyzer, I find the only place I can get low SWR is near
the 15 meter band and some frequency ranges above it, like 6meters and
2 meters. I was hoping the 60 foot length would help me get down to
the lower bands with some compromise, but at least I was hoping to get
there. The transmatch, which consists of 2 capacitors and 2 inductors
isn't any help at all. I've used it in the past to load up random
wires and such, but this project doesn't seem to be working.

Anyone have any experience with this sort of antenna? Let me know
what it is I'm doing wrong.

Any advice is appreciated.

Open Loop aka Dipole bent into a square
 

Priz wrote:
Anyone have any experience with this sort of antenna? Let me know
what it is I'm doing wrong.

One needs to understand how the 300 ohm twinlead
transforms impedances and why a 4:1 balun is probably
not a good choice. One could model this antenna and
feedline with the free demo version of EZNEC
available at eznec.com
--
73, Cecil http://www.w5dxp.com

Open Loop aka Dipole bent into a square
 

I was able to put a 60 foot loop of wire into my attic and am not
having much luck getting it to load up. I configured the loop as an
open loop, aka a dipole bent into a rectangle which is open at one
corner and fed with 300 ohm twinlead at the other. I'm using an MFJ
911H 4:1 and 1:1 balun to tranform the twinlead to coax and a homebrew
antenna tuner.

Using an SWR analyzer, I find the only place I can get low SWR is near
the 15 meter band and some frequency ranges above it, like 6meters and
2 meters. I was hoping the 60 foot length would help me get down to
the lower bands with some compromise, but at least I was hoping to get
there. The transmatch, which consists of 2 capacitors and 2 inductors
isn't any help at all. I've used it in the past to load up random
wires and such, but this project doesn't seem to be working.

Anyone have any experience with this sort of antenna? Let me know
what it is I'm doing wrong.

Any advice is appreciated.
========================================
Instead of using a balun and asymmetric matching unit (transmatch) I
would suggest you use a matching unit to suit a twin feeder directly
,like the EZEE Match or the Link Coupler as per the ARRL Antenna Book of
which I do have the 18th edition ,where the coupler can be found on
pages 25-11 and 25-12.
A variety of matching units to suit twin feeders can also be found on
the web site of PA0FRI.

In your current setup you could use a 1:1 Balun and clip a current
sensor on 1 lead of the twin feeder to determine where there is a
Current maximum for a mid-band frequency . For this purpose you might
have to extend the twin feeder and coil the extension inside the shack.
You would then possibly need a different length coiled up extension for
different bands.
Have a look at Cecil Moore's Web site .

Frank GM0CSZ / KN6WH

Frank GM0CSZ / KN6WH

Open Loop aka Dipole bent into a square
 

"Priz" wrote in message
ups.com...
I'm pressed for space outdoors so right now, I'm working with attic
antennas and need some help.

I was able to put a 60 foot loop of wire into my attic and am not
having much luck getting it to load up. I configured the loop as an
open loop, aka a dipole bent into a rectangle which is open at one
corner and fed with 300 ohm twinlead at the other. I'm using an MFJ
911H 4:1 and 1:1 balun to tranform the twinlead to coax and a homebrew
antenna tuner.

Using an SWR analyzer, I find the only place I can get low SWR is near
the 15 meter band and some frequency ranges above it, like 6meters and
2 meters. I was hoping the 60 foot length would help me get down to
the lower bands with some compromise, but at least I was hoping to get
there. The transmatch, which consists of 2 capacitors and 2 inductors
isn't any help at all. I've used it in the past to load up random
wires and such, but this project doesn't seem to be working.

Anyone have any experience with this sort of antenna? Let me know
what it is I'm doing wrong.

Any advice is appreciated.

Try routing the wire around the attic so that the opposite ends are as far
apart as possible, rather than in a loop. If you have the height, mount the
feedpoint at the apex of the roof and take the wires towards the ends of the
attic. Then bend them around to follow the roof and floor line. I found this
worked more reliably for me than a loop. An ATU feeding a twinline feeder
through a 4:1 balun should be able to load this up on all bands. You could
even try feeding it via an ATU and coax directly provided that you wind a
few turns of the coax feeder around a ferrite rod or ring by the feedpoint
to reduce pickup and RF current on the coax outer. Make sure the ends of the
antenna are well insulated, they will be at high voltage.

Mike G0ULI

Open Loop aka Dipole bent into a square
 

"Priz" wrote in message
ups.com...
I'm pressed for space outdoors so right now, I'm working with attic
antennas and need some help.

I was able to put a 60 foot loop of wire into my attic and am not
having much luck getting it to load up. I configured the loop as an
open loop, aka a dipole bent into a rectangle which is open at one
corner and fed with 300 ohm twinlead at the other. I'm using an MFJ
911H 4:1 and 1:1 balun to tranform the twinlead to coax and a homebrew
antenna tuner.

Using an SWR analyzer, I find the only place I can get low SWR is near
the 15 meter band and some frequency ranges above it, like 6meters and
2 meters. I was hoping the 60 foot length would help me get down to
the lower bands with some compromise, but at least I was hoping to get
there. The transmatch, which consists of 2 capacitors and 2 inductors
isn't any help at all. I've used it in the past to load up random
wires and such, but this project doesn't seem to be working.

Anyone have any experience with this sort of antenna? Let me know
what it is I'm doing wrong.

Any advice is appreciated.

I use a closed loop of approximately the same size. It is corner fed with
coax (don't know the length...probably about 20-25 feet). A MFJ tuner is at
the rig, and will match it all the way down to 80 meters. However,
performance is very bad below 14 Mhz.

So my suggestion would be to first try closing the loop, since this is a
pretty easy experiment with a pair of alligator clips and a piece of wire.
Next, you might change the feedline length to give the tuner a try at
different feedpoint parameters.

I do have a lot of induced RF in the attic wiring. You might use RF chokes
on solid state heat/ac thermostat wiring to prevent damage. Good luck.

Open Loop aka Dipole bent into a square
 

"Priz" wrote in message
ups.com...
I'm pressed for space outdoors so right now, I'm working with attic
antennas and need some help.

I was able to put a 60 foot loop of wire into my attic and am not
having much luck getting it to load up. I configured the loop as an
open loop, aka a dipole bent into a rectangle which is open at one
corner and fed with 300 ohm twinlead at the other. I'm using an MFJ
911H 4:1 and 1:1 balun to tranform the twinlead to coax and a homebrew
antenna tuner.

Using an SWR analyzer, I find the only place I can get low SWR is near
the 15 meter band and some frequency ranges above it, like 6meters and
2 meters. I was hoping the 60 foot length would help me get down to
the lower bands with some compromise, but at least I was hoping to get
there. The transmatch, which consists of 2 capacitors and 2 inductors
isn't any help at all. I've used it in the past to load up random
wires and such, but this project doesn't seem to be working.

Anyone have any experience with this sort of antenna? Let me know
what it is I'm doing wrong.

Any advice is appreciated.

An open loop is supposed to be 1/2 wavelength. A closed loop is supposed to
be 1 wavelength +. A closed loop will sort of work on EVEN harmonics. Using
a 4:1 balun is probably wrong, as is the 300 Ohm line. For 20 meters you
would need about 72 feet of wire in a closed loop, 1:1 balun, and 1/4 WL of
RG11, feeding into a random length of RG8. SWR will be less than 1.5:1 at
resonance. For 40 meters you could use about 65 feet of wire in an open
loop, fed with RG8; haven't tried this, but I think the SWR will be less
than 2:1.

Tam/WB2TT

Open Loop aka Dipole bent into a square
 

I use a closed loop of approximately the same size. It is corner fed with
coax (don't know the length...probably about 20-25 feet). A MFJ tuner is at
the rig, and will match it all the way down to 80 meters. However,
performance is very bad below 14 Mhz.
================================
A closed loop is only effective on wavelengths roughly equal or smaller
than the circumference of the loop , so a 60ft equals 18.3 metres loop
should work well on frequencies 16 MHz or about.

For 14 MHz the loop would be best with a circumference of 21.5 metres
(70 ft) or longer ,etc

Frank GM0CSZ / KN6WH

Open Loop aka Dipole bent into a square
 

On Nov 12, 11:38 am, "Tam/WB2TT" wrote:

An open loop is supposed to be 1/2 wavelength. A closed loop is supposed to
be 1 wavelength +. A closed loop will sort of work on EVEN harmonics. Using
a 4:1 balun is probably wrong, as is the 300 Ohm line. For 20 meters you
would need about 72 feet of wire in a closed loop, 1:1 balun, and 1/4 WL of
RG11, feeding into a random length of RG8. SWR will be less than 1.5:1 at
resonance. For 40 meters you could use about 65 feet of wire in an open
loop, fed with RG8; haven't tried this, but I think the SWR will be less
than 2:1.

Tam/WB2TT

I used 300ohn twinlead because I plan to use this as a multiband
antenna and twinlead wouldn't have as much loss as coax in such a
situation plus I had a big roll of it lying around. The open loop
design was so I could get to a frequency as low as possible with the
amount of space I had. I know I fell a few feet short on 40 meters,
but it was the best I could do under the circumstances. As for the
balun, I bought the switchable balun because I knew some instances
might work better with 4:1, others with 1:1.

I did manage to get the SWR down a little on 40 meters by not using
the balun and just connecting the twinlead with one lead to the single
wire feed and one to the ground, but I had to use almost maximum
inductance and one of the capacitors at max as well, but didn't work
anyone.

The tuner I have was homebrewed by another ham and was built so he
could load up single wire feed antennas for low band and Top Band QRP
work. He told me the capacitors came out of a broadcast receiver and
he found the roller inductor at a hamfest while the second inductor he
made. I used it alot for tuning single wires in the past and it
worked well, although I did have some problems when I went above 17
meters with RF in the shack. Perhaps it was suitable for those kind
of applications, but not for this one and I may need to get a
commecial tuner of figure out how to modify this one to get it to work
better.

I also wonder if the feedline length could have something to do with
the strange characteristics this antenna is exhibiting. It's probably
30 feet or so.

The joys of trying to get on HF in limited space.

Open Loop aka Dipole bent into a square
 

"Priz" wrote in message
ups.com...
On Nov 12, 11:38 am, "Tam/WB2TT" wrote:

An open loop is supposed to be 1/2 wavelength. A closed loop is supposed
to
be 1 wavelength +. A closed loop will sort of work on EVEN harmonics.
Using
a 4:1 balun is probably wrong, as is the 300 Ohm line. For 20 meters you
would need about 72 feet of wire in a closed loop, 1:1 balun, and 1/4 WL
of
RG11, feeding into a random length of RG8. SWR will be less than 1.5:1 at
resonance. For 40 meters you could use about 65 feet of wire in an open
loop, fed with RG8; haven't tried this, but I think the SWR will be less
than 2:1.

Tam/WB2TT

I used 300ohn twinlead because I plan to use this as a multiband
antenna and twinlead wouldn't have as much loss as coax in such a
situation plus I had a big roll of it lying around. The open loop
design was so I could get to a frequency as low as possible with the
amount of space I had. I know I fell a few feet short on 40 meters,
but it was the best I could do under the circumstances. As for the
balun, I bought the switchable balun because I knew some instances
might work better with 4:1, others with 1:1.

I did manage to get the SWR down a little on 40 meters by not using
the balun and just connecting the twinlead with one lead to the single
wire feed and one to the ground, but I had to use almost maximum
inductance and one of the capacitors at max as well, but didn't work
anyone.

The tuner I have was homebrewed by another ham and was built so he
could load up single wire feed antennas for low band and Top Band QRP
work. He told me the capacitors came out of a broadcast receiver and
he found the roller inductor at a hamfest while the second inductor he
made. I used it alot for tuning single wires in the past and it
worked well, although I did have some problems when I went above 17
meters with RF in the shack. Perhaps it was suitable for those kind
of applications, but not for this one and I may need to get a
commecial tuner of figure out how to modify this one to get it to work
better.

I also wonder if the feedline length could have something to do with
the strange characteristics this antenna is exhibiting. It's probably
30 feet or so.

The joys of trying to get on HF in limited space.

30 feet is around 1/4 wavelength at 40 m. So, if the impedance at the
feedpoint is less than 300 Ohms, the twin lead will transform it to a higher
impedance. It will almost certainly work better if you connect the coax
directly to the antenna. With or without a balun.

Tam/WB2TT

Open Loop aka Dipole bent into a square
 

Priz wrote:
I also wonder if the feedline length could have something to do with
the strange characteristics this antenna is exhibiting. It's probably
30 feet or so.

Here's some related information on feedline length:

http://www.w5dxp.com/notuner.htm
--
73, Cecil http://www.w5dxp.com

Open Loop aka Dipole bent into a square
 

Priz wrote:
"I`m working with attic antennas and need some help."

The attic is a poor place for antennas but a good place to experiment.

A 60-ft. perimeter is over one wavelength at 15 meters.

According to Arnold B. Bailey`s "TV and Other Receiving Antennas" Fig.
8-14 on page 408, an open-circuit large loop has a radiation resistance
of less than 80 ohms. A closed-circuit large loop has a radiation
resistance of over 40 ohms.

A closed-loop`s radiation resistance varies less with frequency than
does the open loop.

Borrow the book. Look at the chart. Pick the radiation resistance you
want and proceed.

Best regards, Richard Harrison, KB5WZI

Open Loop aka Dipole bent into a square
 

I've got an update on my antenna experiment. I decided to buy a new
tuner, the MFJ 902H to see if that would help. It was priced low
enough and had a built in balun. I tested it out using my SWR
analyzer and this tuner was able to find matching points in the 80,
40, 30, 20, 17, 15, 12, and 10 meter bands. I did try to find a
matching point on 160 meters but wasn't able to do so which is OK,
since I've never worked 160 and the manual says not to try it on 160
anyway.

I plan to put this antenna on the air over the weekend to see what it
can do. I know it will be a compromise antenna but I'm hoping it will
allow me to do what many other hams in my situation aren't doing,
getting on the air. I also know all the jokes about MFJ, Mighty Fine
Junk, Made from Junk or my personal favorite, Mississippi's Finest
Junk, but it was priced right and when I got it, it appeared to be
well put together. I'll let everyone know how it works out.

Later...

Priz

Open Loop aka Dipole bent into a square
 

On Nov 16, 8:54 am, Priz wrote:
I've got an update on my antenna experiment. I decided to buy a new
tuner, the MFJ 902H to see if that would help. It was priced low
enough and had a built in balun. I tested it out using my SWR
analyzer and this tuner was able to find matching points in the 80,
40, 30, 20, 17, 15, 12, and 10 meter bands. I did try to find a
matching point on 160 meters but wasn't able to do so which is OK,
since I've never worked 160 and the manual says not to try it on 160
anyway.

I plan to put this antenna on the air over the weekend to see what it
can do. I know it will be a compromise antenna but I'm hoping it will
allow me to do what many other hams in my situation aren't doing,
getting on the air. I also know all the jokes about MFJ, Mighty Fine
Junk, Made from Junk or my personal favorite, Mississippi's Finest
Junk, but it was priced right and when I got it, it appeared to be
well put together. I'll let everyone know how it works out.

Later...

Priz

The MFJ tuners are ok as long as you don't abuse them..
And on some , you probably should take the power ratings
with a small bit of salt depending on the load you are matching.
I've got a small MFJ random wire tuner that I've had since 1977.
It still works, and I often use it as a mobile matcher. Also
handy as a "line flattener" when running coax to a solid
state rig.
I've got a 989C that I got free from a guy that fried the coil.
He was ignoring the instructions and running high power
into a nearly half size dipole. I'm talking 8877 power.. :/
Started melting the coil form. Went through two of them.
So he went out and bought a big Nye Viking, and gave
me the 989C as basically junk.. To him anyway..
I messed with it a couple of hours getting the wires
back in the right positions, and then resetting the
roller tension, etc, and got it working again. I've been
using it ever since. Well, when I use a tuner anyway..
I use the watt meter and antenna switch more than
anything.
It's rated 3KW, "dash of salt", but it will handle and
tune about anything I will ever be messing with.
Being it's a T network tuner, it's lousy for feeding
half size dipoles though.. :( It will do 160m though.
MK

Open Loop aka Dipole bent into a square
 

wrote in message
...

snip

The MFJ tuners are ok as long as you don't abuse them..
And on some , you probably should take the power ratings
with a small bit of salt depending on the load you are matching.
I've got a small MFJ random wire tuner that I've had since 1977.
It still works, and I often use it as a mobile matcher. Also
handy as a "line flattener" when running coax to a solid
state rig.

snip

I picked up a used MFJ tuner at a local ham swap meet. As is my wont, I
took off the cover to give it the visual once-over before dumping any 'trons
into it. Good thing. About six mounting and/or connection points - screws
or nut/bolt combos -- were so loose as to guarantee problems.

It's anybody's guess how long that stuff needed tightening. Was it ever
tight? (hint-hint)

"Sal"

PS: The thing works pretty good, overall.

Open Loop aka Dipole bent into a square
 

"Sal M. Onella" wrote in message
...
snip

I picked up a used MFJ tuner at a local ham swap meet. As is my wont, I
took off the cover to give it the visual once-over before dumping any
'trons
into it. Good thing. About six mounting and/or connection points -
screws
or nut/bolt combos -- were so loose as to guarantee problems.

It's anybody's guess how long that stuff needed tightening. Was it ever
tight? (hint-hint)

"Sal"

PS: The thing works pretty good, overall.

The few MFJ items I have remind me of an old Ford Bronco II that I once
owned. It was a piece of junk from day one, extremely useful, and reliable.
With a little TLC it ran forever, and was the second best car I ever had.
For example, my old MFJ CW audio filter is great, and has given over 20
years of service. But you don't want to look inside at the construction.

Open Loop aka Dipole bent into a square
 

I put the antenna on the air last night and this morning and did make
a couple of CW QSOs. From Louisiana, I worked Virginia on 40meters
with around 40 watts and Missorui on 30meters with 5 watts. Not
great, but I'm getting on the air!

Open Loop aka Dipole bent into a square
 

Priz wrote:
I've got an update on my antenna experiment. I decided to buy a new
tuner, the MFJ 902H to see if that would help. It was priced low
enough and had a built in balun. I tested it out using my SWR
analyzer and this tuner was able to find matching points in the 80,
40, 30, 20, 17, 15, 12, and 10 meter bands. I did try to find a
matching point on 160 meters but wasn't able to do so which is OK,
since I've never worked 160 and the manual says not to try it on 160
anyway.
=========================
You highly likely find a matching point for the 160 m band when you
extend the twin feeder . The extension length to be such that from any
end of the dipole to the matching unit the total length is roughly 1 or
3 or 5 etc quarter wavelengths .Any part of that feeder which is inside
the shack can be coiled -up (kept away from a metal clad area).

Frank GM0CSZ / KN6WH

Open Loop aka Dipole bent into a square
 

Highland Ham wrote:
You highly likely find a matching point for the 160 m band when you
extend the twin feeder .

Sorry, with a feedpoint impedance with R in the units
and X in the thousands, there is probably no length of
feedline that will come close to being compatible with
a tuner. No matter what feedline is used the SWR is way
up in the hundreds to thousands.
--
73, Cecil http://www.w5dxp.com