Trap dipole
 

Anybody tried to build this trap antenna yet?

http://www.nerc.com/~jdegood/coaxtrap/
http://members.shaw.ca/ve6yp/
http://members.fortunecity.com/xe1bef/hf-antennas.htm

Need some help on where to tune the traps for the bands of operation, and
pruning the connection wiring. Can this be done with out a major test
equipment investment?

Yes, I agree. I have used open wire dipoles and they work great over all
bands. But, my problem is constrained by wife and housing subdivision
requirements. So my antenna is confined to the attic. I tried to use a short
version of the open wire dipole and it is not loading at all and the RFI is
everywhere. So I guess traps are my only option at this point. I just never
built one before or had experience with coaxial type traps either. Just need
to know where to tune the traps for maximum impedance on the band of
operation. Your explanation of pruning makes sense.

Thanks for your reply
73, Bill


"WB3FUP (Mike Hall)" wrote in message
...
When building a Trap dipole you start by building from the inside out.
Lets say you want an dipole for 20 and 40. Cut a twenty meter dipole,
hang
it up and adjust it for minimum SWR. Then add the traps and balance of
the
antenna. You will obtain some loading from the coils in the traps so the
40 meter antenna will be somewhat shorter than you would expect. Prune
for
operation on 40 meters. The problem with traps is that you need a set for
each band you want to work. You can't sneak by on the odd multiple
halfwave, the traps might be at a bad place on that 20/40 antenna used on
15. I have a great example of that on a trapped dipole I loaded on 12 to
try the band out..

I regret the years wasted with coaxial fed dipoles. Life is much easier
if
you hang as much wire as you can, as high as you can get it, and feed with
ladder line. I work all 80 through 10 with a 68' (34' a side) dipole fed
with 460 ohm ladder line. For portable use I have 75 feet of AWG#22
speaker wire. 34" is split and forms the doublet, the rest is the
transmission line. It is usable from 80 to 10 and works great.

--
73 es cul

wb3fup
a Salty Bear

"Bill" wrote in message
...
Anybody tried to build this trap antenna yet?

http://www.nerc.com/~jdegood/coaxtrap/
http://members.shaw.ca/ve6yp/
http://members.fortunecity.com/xe1bef/hf-antennas.htm

Need some help on where to tune the traps for the bands of operation,
and
pruning the connection wiring. Can this be done with out a major test
equipment investment?

The RFI is probably due to the fact that your antenna is so close to the
affected equipment. I used to make the bathroom flourescent light go on
with my attic dipole.

A trap design will have its current nodes in different places than a simple
dipole, but if it is working, the field strength in the house will be the
same.

The ARRL antenna book should show you where to tune the traps.


Alternativly:
If your tuner (via balanced line, NOT coax) would not tune the antenna on
multi bands, try making it into a loop. Follow around the house with the
largest size you can. Just staple the wire to the rafters. Don't worry if
your tuner does not have a balanced output, with the loop it won't make much
difference. The "balun" in the tuner would just add losses.

My roof has shingles made from Steel Slag, not gravel. I suspect it has a
much different RF charectaristic than the stone shingles do.

KA9CAR
John

"Bill" wrote in message
. ..
Yes, I agree. I have used open wire dipoles and they work great over all
bands. But, my problem is constrained by wife and housing subdivision
requirements. So my antenna is confined to the attic. I tried to use a
short
version of the open wire dipole and it is not loading at all and the RFI
is
everywhere. So I guess traps are my only option at this point. I just
never
built one before or had experience with coaxial type traps either. Just
need
to know where to tune the traps for maximum impedance on the band of
operation. Your explanation of pruning makes sense.

Thanks for your reply
73, Bill


"WB3FUP (Mike Hall)" wrote in message
...
When building a Trap dipole you start by building from the inside out.
Lets say you want an dipole for 20 and 40. Cut a twenty meter dipole,
hang
it up and adjust it for minimum SWR. Then add the traps and balance of
the
antenna. You will obtain some loading from the coils in the traps so
the
40 meter antenna will be somewhat shorter than you would expect. Prune
for
operation on 40 meters. The problem with traps is that you need a set
for
each band you want to work. You can't sneak by on the odd multiple
halfwave, the traps might be at a bad place on that 20/40 antenna used
on
15. I have a great example of that on a trapped dipole I loaded on 12
to
try the band out..

I regret the years wasted with coaxial fed dipoles. Life is much easier
if
you hang as much wire as you can, as high as you can get it, and feed
with
ladder line. I work all 80 through 10 with a 68' (34' a side) dipole
fed
with 460 ohm ladder line. For portable use I have 75 feet of AWG#22
speaker wire. 34" is split and forms the doublet, the rest is the
transmission line. It is usable from 80 to 10 and works great.

--
73 es cul

wb3fup
a Salty Bear

"Bill" wrote in message
...
Anybody tried to build this trap antenna yet?

http://www.nerc.com/~jdegood/coaxtrap/
http://members.shaw.ca/ve6yp/
http://members.fortunecity.com/xe1bef/hf-antennas.htm

Need some help on where to tune the traps for the bands of operation,
and
pruning the connection wiring. Can this be done with out a major test
equipment investment?

On Wed, 06 Aug 2003 21:59:37 GMT, "Bill"
wrote:

Anybody tried to build this trap antenna yet?

http://www.nerc.com/~jdegood/coaxtrap/
http://members.shaw.ca/ve6yp/
http://members.fortunecity.com/xe1bef/hf-antennas.htm

Need some help on where to tune the traps for the bands of operation, and
pruning the connection wiring. Can this be done with out a major test
equipment investment?

I love those coax traps. About 20 years ago, I spent several hours
with a sweep generator and scalar network analyzer building a set
of traps for 40/20/15/10 meters. Then, I spent almost all day putting
up the antenna. The process is basically "put up one section, check
swr, drop antenna, trim antenna, raise it back up, and repeat until
the SWR is acceptable" (typical dipole scenario). Then, however, you
add a set of traps and the next section, and repeat the entire process
for the next lower (in frequency) band. Keep repeating till you have
all bands covered. Got a nice sunburn and a decent antenna out of the
deal.

The traps can be fairly light weight and compact - I used RG-174 and
wound 'em on empty pill bottles. For test equipment, a plain signal
generator and oscilloscope will suffice, or maybe even just a grid-dip
meter.... Heck - I even got so involved with coax traps that I wrote
a computer program to characterize the things and tell me how many
turns of what kind of coax on what size form would be resonant at
my frequency of interest.

Since that time, I've discovered how pointless it was. Like WB3FUP
suggests, just put up as much wire as you can, as high as you can.
Right now, I'm using a 130 foot inverted L, and it works every bit
as good as the trap dipole did (if not better) from 160 through
10 meters - and it was a *lot* easier to put up. The only drawback
is that you gotta have a tuner.

-fb-

I've built a number of antennas with coax traps made from both RG-174
and RG-58. There are a couple of ways to connect the traps, with one
(sort of cross-connecting the ends) giving a more favorable L/C ratio
than the other. But after measuring the trap L, C, and Q and doing some
EZNEC modeling, I discovered that it's often hard, even with the RG-58
versions, to get low loss on all bands the antenna is intended for. The
loss might be low on the band the trap "traps", but relatively high
(typically a couple of dB) on a lower band. Or vice-versa, depending on
the combination of bands that the antenna is for. It depends on the
combination of bands that the antenna is for -- some are ok, some not so
good. The fact is that the effective inductor Q of coax traps made with
RG-58 or smaller coax is mediocre.

For a lot of applications, a couple of dB is an acceptable trade for the
simplicity of coax traps, but for some it's not. If it's not, I'd
recommend doing some modeling, or careful comparison to a non-trapped
antenna.

One other word of advice is to make sure you weatherproof the traps in a
way that prevents any water from getting between the turns. Water
between the turns can increase trap loss substantially.

Roy Lewallen, W7EL

funkbastler wrote:
On Wed, 06 Aug 2003 21:59:37 GMT, "Bill"
wrote:


Anybody tried to build this trap antenna yet?

http://www.nerc.com/~jdegood/coaxtrap/
http://members.shaw.ca/ve6yp/
http://members.fortunecity.com/xe1bef/hf-antennas.htm

Need some help on where to tune the traps for the bands of operation, and
pruning the connection wiring. Can this be done with out a major test
equipment investment?


I love those coax traps. About 20 years ago, I spent several hours
with a sweep generator and scalar network analyzer building a set
of traps for 40/20/15/10 meters. Then, I spent almost all day putting
up the antenna. The process is basically "put up one section, check
swr, drop antenna, trim antenna, raise it back up, and repeat until
the SWR is acceptable" (typical dipole scenario). Then, however, you
add a set of traps and the next section, and repeat the entire process
for the next lower (in frequency) band. Keep repeating till you have
all bands covered. Got a nice sunburn and a decent antenna out of the
deal.

The traps can be fairly light weight and compact - I used RG-174 and
wound 'em on empty pill bottles. For test equipment, a plain signal
generator and oscilloscope will suffice, or maybe even just a grid-dip
meter.... Heck - I even got so involved with coax traps that I wrote
a computer program to characterize the things and tell me how many
turns of what kind of coax on what size form would be resonant at
my frequency of interest.

Since that time, I've discovered how pointless it was. Like WB3FUP
suggests, just put up as much wire as you can, as high as you can.
Right now, I'm using a 130 foot inverted L, and it works every bit
as good as the trap dipole did (if not better) from 160 through
10 meters - and it was a *lot* easier to put up. The only drawback
is that you gotta have a tuner.

-fb-

Hi Bill -

I tried to find the specific antenna you are considering using your links,
but I wasn't able to do so with a reasonable amount of effort.

However, there is no doubt at all that a well-designed trap antenna
could do a good job for you.

I am rather bemused by the responses you got suggesting a one-size-fits-all
ladder-line doublet instead. I'm sure these can be good antennas; but
they usually seem to be presented by their advocates as having no draw-
backs at all. That would be an incorrect view, as those antennas have
their own tradeoffs--just like other antennas.

The biggest tradeoff is the tuner, of course. If you're planning on running
power, this is about a $600 +/- item (commercial models), and it takes
up space at your operating position. But you're not done when you buy
it and find a space for it... you also have to TUNE it every time you make
a significant QSY. (I won't mention tuner losses unless someone brings
up trap losses.)

I friend of mine took a look at my array of two W9INN trap/fan dipoles
(6-band coverage); but he decided that he wanted to go with the ubiquitous
135-foot doublet-plus-tuner. (He bought the tuner from me, which was
great... I don't need it!)

When I asked him how he was doing with his new setup, he reported
that it gets out fine on 80m and 40m, but is such a pain in the a*s to
tune on the higher bands that he doesn't bother with them. (That's just
his experience, other users seem to have less trouble.) Oh, and if he
ever decides to run power, he'll have to buy a BIGGER tuner, and
probably re-sell the low-power tuner he bought from me.

Meanwhile, I'm enjoying six bands, instant QSY, no tuner needed, and
I run power when I want to. Because of the loading effects of the traps,
my antennas fit in a very compact space... which is all the space I've got.
I hear well,and my reports are excellent. Not bad, not bad at all.
All I really want that I haven't got is the ability to get these antennas
straighter and HIGHER.

Good luck with your selection, and I hope whatever you choose works
great for you.

73,

Ed W6LOL

"Bill" wrote in message
...
Anybody tried to build this trap antenna yet?

http://www.nerc.com/~jdegood/coaxtrap/
http://members.shaw.ca/ve6yp/
http://members.fortunecity.com/xe1bef/hf-antennas.htm

Need some help on where to tune the traps for the bands of operation, and
pruning the connection wiring. Can this be done with out a major test
equipment investment?

Ed Senior wrote:
I am rather bemused by the responses you got suggesting a one-size-fits-all
ladder-line doublet instead. I'm sure these can be good antennas; but
they usually seem to be presented by their advocates as having no draw-
backs at all. That would be an incorrect view, as those antennas have
their own tradeoffs--just like other antennas.

The biggest tradeoff is the tuner, of course.

I use the transmission line to tune my 130 ft. dipole, no tuner needed.

Meanwhile, I'm enjoying six bands, instant QSY, no tuner needed, and
I run power when I want to.

I'm enjoying eight bands, throw a couple of knife switches to QSY,
no tuner needed, and I run power when I want to. Also no messing
with the interaction of multiple legs of dipoles.
--
73, Cecil http://www.qsl.net/w5dxp

Hi Cecil -

I'm glad your antenna is working well for you!
A few more comments below...

"W5DXP" wrote in message
...
Ed Senior wrote:
I am rather bemused by the responses you got suggesting a
one-size-fits-all
ladder-line doublet instead. I'm sure these can be good antennas; but
they usually seem to be presented by their advocates as having no draw-
backs at all. That would be an incorrect view, as those antennas have
their own tradeoffs--just like other antennas.

The biggest tradeoff is the tuner, of course.

I use the transmission line to tune my 130 ft. dipole, no tuner needed.

Well, perhaps I should have said something like

"...no tuner needed, implemented either as the traditional box with
tunable L and C, OR implemented in distributed fashion as a switchable-
length transmission-line transformer, usually including a current balun."

But that would have been a heck of a mouthful; and most of the users
of this antenna type seem to advocate the traditional form of the tuner.

Actually, I rather LIKE the switchable-length transmission line idea.
Could you refresh me on many different lengths do you find you need to
be able to get the SWR below, say, 2:1 at every point in those 8 bands?
(I think we talked about this in an earlier thread, but I don't remember
all the details.) How do you stow the tuning lengths, and how many
physical switches (or plug-ins) are needed? When you do a typical
band change, does "a couple" of switch operations literally mean two,
or several? Any safety issues with the switches if you're running power?

Meanwhile, I'm enjoying six bands, instant QSY, no tuner needed, and
I run power when I want to.

I'm enjoying eight bands, throw a couple of knife switches to QSY,
no tuner needed, and I run power when I want to. Also no messing
with the interaction of multiple legs of dipoles.

I would only add that I found the W9INN dipoles to be extremely easy
to tune, with no troublesome interactions. Also, there is no "messing"
with either a traditional tuner, or switching a tunable transmission-line
transformer.

--
73, Cecil http://www.qsl.net/w5dxp

Gee I spent not more than 2 hours with my MFJ259, my MFJ Tuner (I think it
cost me 129.95 and is good for 300 watts, which is three times the power I
will ever need, and 600 times the power that I used most of the time), and
a pencil and paper. I am old fashioned and maintain a log. In the log I
have recorded tuner settings for each of the ham bands. (75 and 80 are
separate bands) QSY is a matter of referring to log, presetting dials, and
changing bands. If I feel the burning need a quick touch up of the
capacitor and I am matched. My transceiver, and the DIP unit sit side by
side on top of the tuner, so there is no significant footprint problem at
the operating position.

You failed to mention the only major drawback to the ladder line fed
doublet that I am aware of. It develops some strange lobes as it gets
longer. I have heard that the 44' length of the NorCal "Crappie" dipole
was chosen as a compromise between 80 meter efficiency and 10 meter lobes.
I have been scared of "fan" dipoles for years. I worry that given a
reasonable radiator I put enough 7.810 signal on the air to upset the FCC
when I was on 3.905. Of course the tuner you seem to refuse to run would
help with that somewhat, but I still do not like having the nearly resonate
antenna connected to the transmitter any way. I have found out just how
far I can talk with a couple of watts (halfway around the world, after that
it is shorter going the other way). I never have run an amplifier, there
was a time I put 225 plus watts out on 80, and 40 (TR4) but since I got my
first ICOM never more than 100, seldom that much. Still worried about
harmonics where I have no business radiating, and the FT817 has
dramatically demonstrated just how little power is required for a QSO.
Amplify the harmonic, and couple it to a nearly resonate antenna, no thank
you, I like my license to much, and do not have all that much money for
fines laying around the house.

--
73 es cul

wb3fup
a Salty Bear

"Ed Senior" wrote in message
nk.net...
Hi Bill -

I tried to find the specific antenna you are considering using your
links,
but I wasn't able to do so with a reasonable amount of effort.

However, there is no doubt at all that a well-designed trap antenna
could do a good job for you.

I am rather bemused by the responses you got suggesting a
one-size-fits-all
ladder-line doublet instead. I'm sure these can be good antennas; but
they usually seem to be presented by their advocates as having no draw-
backs at all. That would be an incorrect view, as those antennas have
their own tradeoffs--just like other antennas.

The biggest tradeoff is the tuner, of course. If you're planning on
running
power, this is about a $600 +/- item (commercial models), and it takes
up space at your operating position. But you're not done when you buy
it and find a space for it... you also have to TUNE it every time you
make
a significant QSY. (I won't mention tuner losses unless someone brings
up trap losses.)

I friend of mine took a look at my array of two W9INN trap/fan dipoles
(6-band coverage); but he decided that he wanted to go with the
ubiquitous
135-foot doublet-plus-tuner. (He bought the tuner from me, which was
great... I don't need it!)

When I asked him how he was doing with his new setup, he reported
that it gets out fine on 80m and 40m, but is such a pain in the a*s to
tune on the higher bands that he doesn't bother with them. (That's just
his experience, other users seem to have less trouble.) Oh, and if he
ever decides to run power, he'll have to buy a BIGGER tuner, and
probably re-sell the low-power tuner he bought from me.

Meanwhile, I'm enjoying six bands, instant QSY, no tuner needed, and
I run power when I want to. Because of the loading effects of the traps,
my antennas fit in a very compact space... which is all the space I've
got.
I hear well,and my reports are excellent. Not bad, not bad at all.
All I really want that I haven't got is the ability to get these antennas
straighter and HIGHER.

Good luck with your selection, and I hope whatever you choose works
great for you.

73,

Ed W6LOL

"Bill" wrote in message
...
Anybody tried to build this trap antenna yet?

http://www.nerc.com/~jdegood/coaxtrap/
http://members.shaw.ca/ve6yp/
http://members.fortunecity.com/xe1bef/hf-antennas.htm

Need some help on where to tune the traps for the bands of operation,
and
pruning the connection wiring. Can this be done with out a major test
equipment investment?

I tried to find the specific antenna you are considering using
your links, but I wasn't able to do so with a reasonable amount
of effort.

Sorry, my ISP recently ceased operation. You can find the page
originally referenced, at least in the near term, at:

http://users.tellurian.com/jdegood/coaxtrap/

It's simply a six-band trap dipole. The 5 pairs of coaxial-cable traps
provide significant physical shortening, although to fit in an attic
some or all of the outermost (80 meter) portion will probably have to be
run at right angles to the main dipole. The advantages of such a
resonant antenna include a decent match to a coax feedline and no-tune
operation (but over limited bandwidth, a side-effect of the reduced size.)

Another alternative for multi-band operation in a restricted space
situation such as an attic is a physically shortened dipole in
conjunction with a tuner and open wire feedline. For details, see:

http://www.arrl.org/tis/info/HTML/li.../limited1.html

73,

John NU3E