On 11/22/2014 4:09 PM, Wayne wrote:


"John S" wrote in message ...

On 11/22/2014 10:19 AM, John S wrote:
A special purpose antenna for maybe just below the 6M band. For railroad
cars (a large metal ground plane). The antenna must be short enough to
pass through tunnels and have a 50 ohm feed impedance.

I saw this antenna in a book but I can no longer remember which book
and, although I've searched, I can't seem to find a reference. It was
probably from the 1950's. Anyway...

Imagine a folded unipole over a large sheet of metal. It will probably
have a high feed resistance of 100 or so ohms. But, if it is bent over
90 degrees starting a short distance above the ground plane, it can be
adjusted to match a 50 ohm feed and with no imaginary component. This
will satisfy not only the feed impedance but also the short height
requirement.

Other than the really nice ground plane of a railroad car's roof and
using a frequency proportional to the plane, there is no obvious reason
this cannot be use in other situations.

Is that not really cool? Comments welcome, of course.

# No, Guys, nothing that I have read so far is the thing I have in mind.

# Picture this...

# A folded unipole on a large sheet of conductive material. Let one of the
# vertical elements be connected to the sheet. The other is insulated from
# the sheet and has the generator attached.

# Now, this configuration has a high feed impedance, even when resonant.
# However, if you now bend the antenna at an appropriate point vertically
# so that the top portion leans horizontally to the sheet, the antenna
# gets lower to the sheet and the impedance lowers.

# I have an EZNEC file that I can share if anyone is interested. I will
# also continue to search my books for the example.

Ahhh....the "folded" part didn't sink in when I responded.

I suspect my original comments still apply.
With two conductors, the current is half in each conductor, thus a
doubling of radiation resistance at the feed point in one conductor.
So by carefully selecting the height at which the vertical part is bent,
there might be a good match for coax.

I'll play with that configuration a bit on EZNEC.

If you have AutoEZ to go with EZNEC, it is easy to find the bending point.

"John S" wrote in message ...

On 11/22/2014 10:19 AM, John S wrote:
A special purpose antenna for maybe just below the 6M band. For railroad
cars (a large metal ground plane). The antenna must be short enough to
pass through tunnels and have a 50 ohm feed impedance.

I saw this antenna in a book but I can no longer remember which book
and, although I've searched, I can't seem to find a reference. It was
probably from the 1950's. Anyway...

Imagine a folded unipole over a large sheet of metal. It will probably
have a high feed resistance of 100 or so ohms. But, if it is bent over
90 degrees starting a short distance above the ground plane, it can be
adjusted to match a 50 ohm feed and with no imaginary component. This
will satisfy not only the feed impedance but also the short height
requirement.

Other than the really nice ground plane of a railroad car's roof and
using a frequency proportional to the plane, there is no obvious reason
this cannot be use in other situations.

Is that not really cool? Comments welcome, of course.

# No, Guys, nothing that I have read so far is the thing I have in mind.

# Picture this...
snip

# I have an EZNEC file that I can share if anyone is interested. I will
# also continue to search my books for the example.

If it isn't too much trouble, I'd like to see the EZNEC file. My email
address on the post is correct.

On 11/23/2014 11:16 AM, Wayne wrote:


"John S" wrote in message ...

On 11/22/2014 10:19 AM, John S wrote:
A special purpose antenna for maybe just below the 6M band. For railroad
cars (a large metal ground plane). The antenna must be short enough to
pass through tunnels and have a 50 ohm feed impedance.

I saw this antenna in a book but I can no longer remember which book
and, although I've searched, I can't seem to find a reference. It was
probably from the 1950's. Anyway...

Imagine a folded unipole over a large sheet of metal. It will probably
have a high feed resistance of 100 or so ohms. But, if it is bent over
90 degrees starting a short distance above the ground plane, it can be
adjusted to match a 50 ohm feed and with no imaginary component. This
will satisfy not only the feed impedance but also the short height
requirement.

Other than the really nice ground plane of a railroad car's roof and
using a frequency proportional to the plane, there is no obvious reason
this cannot be use in other situations.

Is that not really cool? Comments welcome, of course.

# No, Guys, nothing that I have read so far is the thing I have in mind.

# Picture this...
snip

# I have an EZNEC file that I can share if anyone is interested. I will
# also continue to search my books for the example.

If it isn't too much trouble, I'd like to see the EZNEC file. My email
address on the post is correct.


No trouble at all, I think. Never tried this before, so let me know if
it is not successful. By the way, it is for my frequency of interest,
434MHz. If that is a problem, I can scale it for you.

"John S" wrote in message ...

On 11/23/2014 11:16 AM, Wayne wrote:


"John S" wrote in message ...

On 11/22/2014 10:19 AM, John S wrote:
A special purpose antenna for maybe just below the 6M band. For railroad
cars (a large metal ground plane). The antenna must be short enough to
pass through tunnels and have a 50 ohm feed impedance.

I saw this antenna in a book but I can no longer remember which book
and, although I've searched, I can't seem to find a reference. It was
probably from the 1950's. Anyway...

Imagine a folded unipole over a large sheet of metal. It will probably
have a high feed resistance of 100 or so ohms. But, if it is bent over
90 degrees starting a short distance above the ground plane, it can be
adjusted to match a 50 ohm feed and with no imaginary component. This
will satisfy not only the feed impedance but also the short height
requirement.

Other than the really nice ground plane of a railroad car's roof and
using a frequency proportional to the plane, there is no obvious reason
this cannot be use in other situations.

Is that not really cool? Comments welcome, of course.

# No, Guys, nothing that I have read so far is the thing I have in mind.

# Picture this...
snip

# I have an EZNEC file that I can share if anyone is interested. I will
# also continue to search my books for the example.

If it isn't too much trouble, I'd like to see the EZNEC file. My email
address on the post is correct.


# No trouble at all, I think. Never tried this before, so let me know if
# it is not successful. By the way, it is for my frequency of interest,
# 434MHz. If that is a problem, I can scale it for you.

Got it, thanks.

It is as I had envisioned originally, but without the second wire.

Scaling it to 7 MHz, the 3:1 SWR bandwidth is about 400 KHz, and the antenna
is about 12 feet high.
Over "real" ground, there is some pattern skew. And the feedpoint is not a
bad match for 50 ohm cable.

This is an interesting antenna solution if antenna height is a
consideration.

On 11/23/2014 12:29 PM, Wayne wrote:


"John S" wrote in message ...

On 11/23/2014 11:16 AM, Wayne wrote:


"John S" wrote in message ...

On 11/22/2014 10:19 AM, John S wrote:
A special purpose antenna for maybe just below the 6M band. For railroad
cars (a large metal ground plane). The antenna must be short enough to
pass through tunnels and have a 50 ohm feed impedance.

I saw this antenna in a book but I can no longer remember which book
and, although I've searched, I can't seem to find a reference. It was
probably from the 1950's. Anyway...

Imagine a folded unipole over a large sheet of metal. It will probably
have a high feed resistance of 100 or so ohms. But, if it is bent over
90 degrees starting a short distance above the ground plane, it can be
adjusted to match a 50 ohm feed and with no imaginary component. This
will satisfy not only the feed impedance but also the short height
requirement.

Other than the really nice ground plane of a railroad car's roof and
using a frequency proportional to the plane, there is no obvious reason
this cannot be use in other situations.

Is that not really cool? Comments welcome, of course.

# No, Guys, nothing that I have read so far is the thing I have in mind.

# Picture this...
snip

# I have an EZNEC file that I can share if anyone is interested. I will
# also continue to search my books for the example.

If it isn't too much trouble, I'd like to see the EZNEC file. My email
address on the post is correct.


# No trouble at all, I think. Never tried this before, so let me know if
# it is not successful. By the way, it is for my frequency of interest,
# 434MHz. If that is a problem, I can scale it for you.

Got it, thanks.

It is as I had envisioned originally, but without the second wire.

Scaling it to 7 MHz, the 3:1 SWR bandwidth is about 400 KHz, and the
antenna is about 12 feet high.
Over "real" ground, there is some pattern skew. And the feedpoint is
not a bad match for 50 ohm cable.

This is an interesting antenna solution if antenna height is a
consideration.

As you may know, I have not applied the idea to anything less than about
50MHz, so you are on you own there. You might need lots of ground
radials. This outside my area of knowledge, for sure.

Wayne wrote:


"John S" wrote in message ...

On 11/23/2014 11:16 AM, Wayne wrote:


"John S" wrote in message ...

On 11/22/2014 10:19 AM, John S wrote:
A special purpose antenna for maybe just below the 6M band. For railroad
cars (a large metal ground plane). The antenna must be short enough to
pass through tunnels and have a 50 ohm feed impedance.

I saw this antenna in a book but I can no longer remember which book
and, although I've searched, I can't seem to find a reference. It was
probably from the 1950's. Anyway...

Imagine a folded unipole over a large sheet of metal. It will probably
have a high feed resistance of 100 or so ohms. But, if it is bent over
90 degrees starting a short distance above the ground plane, it can be
adjusted to match a 50 ohm feed and with no imaginary component. This
will satisfy not only the feed impedance but also the short height
requirement.

Other than the really nice ground plane of a railroad car's roof and
using a frequency proportional to the plane, there is no obvious reason
this cannot be use in other situations.

Is that not really cool? Comments welcome, of course.

# No, Guys, nothing that I have read so far is the thing I have in mind.

# Picture this...
snip

# I have an EZNEC file that I can share if anyone is interested. I will
# also continue to search my books for the example.

If it isn't too much trouble, I'd like to see the EZNEC file. My email
address on the post is correct.


# No trouble at all, I think. Never tried this before, so let me know if
# it is not successful. By the way, it is for my frequency of interest,
# 434MHz. If that is a problem, I can scale it for you.

Got it, thanks.

It is as I had envisioned originally, but without the second wire.

Scaling it to 7 MHz, the 3:1 SWR bandwidth is about 400 KHz, and the antenna
is about 12 feet high.
Over "real" ground, there is some pattern skew. And the feedpoint is not a
bad match for 50 ohm cable.

This is an interesting antenna solution if antenna height is a
consideration.

I just ran the thing through the optimizer at 7.15 MHz and got dimensions
(in wavelengths) of .116 high, .132 to the feeder stub, and .14 for the
length of the radiator AFTER the stub, or a total of .272 wavelengths
long.

For very good, average, and very poor ground I got:

Very good
1.9 dBi @ 25 degrees

Average
..3 dBi @ 31 degrees

Very poor
-1.1 dBi @ 35 degrees


The SWR was less than 1.2:1 across the entire band for all cases.

I get basically omniadirectional with a slight skew of greater gain
in the direction oposite the radiator direction.

At about 38 feet long it should fit in most suburban lots, and at about
16 feet tall it is above everyone's head.

One would probably want to put at least some short radials at the feed
point.


--
Jim Pennino

wrote in message ...

Wayne wrote:


"John S" wrote in message ...

On 11/23/2014 11:16 AM, Wayne wrote:


"John S" wrote in message ...

On 11/22/2014 10:19 AM, John S wrote:
A special purpose antenna for maybe just below the 6M band. For railroad
cars (a large metal ground plane). The antenna must be short enough to
pass through tunnels and have a 50 ohm feed impedance.

I saw this antenna in a book but I can no longer remember which book
and, although I've searched, I can't seem to find a reference. It was
probably from the 1950's. Anyway...

Imagine a folded unipole over a large sheet of metal. It will probably
have a high feed resistance of 100 or so ohms. But, if it is bent over
90 degrees starting a short distance above the ground plane, it can be
adjusted to match a 50 ohm feed and with no imaginary component. This
will satisfy not only the feed impedance but also the short height
requirement.

Other than the really nice ground plane of a railroad car's roof and
using a frequency proportional to the plane, there is no obvious reason
this cannot be use in other situations.

Is that not really cool? Comments welcome, of course.

# No, Guys, nothing that I have read so far is the thing I have in mind.

# Picture this...
snip

# I have an EZNEC file that I can share if anyone is interested. I will
# also continue to search my books for the example.

If it isn't too much trouble, I'd like to see the EZNEC file. My email
address on the post is correct.


# No trouble at all, I think. Never tried this before, so let me know if
# it is not successful. By the way, it is for my frequency of interest,
# 434MHz. If that is a problem, I can scale it for you.

Got it, thanks.

It is as I had envisioned originally, but without the second wire.

Scaling it to 7 MHz, the 3:1 SWR bandwidth is about 400 KHz, and the
antenna
is about 12 feet high.
Over "real" ground, there is some pattern skew. And the feedpoint is not
a
bad match for 50 ohm cable.

This is an interesting antenna solution if antenna height is a
consideration.

# I just ran the thing through the optimizer at 7.15 MHz and got dimensions
# (in wavelengths) of .116 high, .132 to the feeder stub, and .14 for the
# length of the radiator AFTER the stub, or a total of .272 wavelengths
# long.

# For very good, average, and very poor ground I got:

# Very good
# 1.9 dBi @ 25 degrees

# Average
# .3 dBi @ 31 degrees

# Very poor
# -1.1 dBi @ 35 degrees


# The SWR was less than 1.2:1 across the entire band for all cases.

# I get basically omniadirectional with a slight skew of greater gain
# in the direction oposite the radiator direction.

# At about 38 feet long it should fit in most suburban lots, and at about
# 16 feet tall it is above everyone's head.

# One would probably want to put at least some short radials at the feed
# point.

Interesting. I might look into this a bit.

My situation: small lot, 2 story house that is taller than the surrounding
trees on a sloping lot. There is no good place to string any kind of
dipole.
But...
The metal roofed patio cover is about 30 feet wide and 12 feet deep.

I have a 16 foot whip mounted on the patio cover, and use a tuner in the
shack for multiband operation.
I'm considering a remote ATU mounted at the base for easier matching.

The folded unipole might be an alternative for 7 MHz.

Wayne wrote:

snip

Interesting. I might look into this a bit.

My situation: small lot, 2 story house that is taller than the surrounding
trees on a sloping lot. There is no good place to string any kind of
dipole.
But...
The metal roofed patio cover is about 30 feet wide and 12 feet deep.

I have a 16 foot whip mounted on the patio cover, and use a tuner in the
shack for multiband operation.
I'm considering a remote ATU mounted at the base for easier matching.

The folded unipole might be an alternative for 7 MHz.

My feeling is that the remote ATU is the best thing to come along since
sliced bread and bottled beer.

I have a 33 foot vertical in the back yard with an ATU at the base.

It works very well on 80 - 15. Above that it squirts a lot of energy
into the clouds and below that there are losses in the ATU on both
80 and 160.

To mitigate those losses, I put in a relay controlled, high Q loading
coil.

Nothing to be done for the upper frequencies other than maybe a shorter
parallel vertical.



--
Jim Pennino

On Sunday, November 23, 2014 10:38:59 PM UTC+11, Jeff wrote:
The antenna I described is essentially a 1/4 wave vertical bent over
with the bent portion parallel to the ground.
My 2 meter version could be described as a 19 inch long vertical, bent
over two inches from the ground, and the remaining 17 inches formed into
a circle parallel to the ground. So the antenna only added 2 inches to
the height of my van.



Such a Blade antenna was described in the RSGB VHF/UHF Manual. It
consisted of a 1/4 wavelength rod bent over to be parallel with the
ground plane. It is fed with a tap at about 1/3 of the length and tuned
by a screw in the end of the whip.

Also described is a lambda/2 Ring antenna mounted just above a ground plane.

Jeff
This type of antenna is common on aircraft for 75MHz Marker reception. It is folded back and covered with a radome. It has a cardioid pattern with a deep null towards the "front" (for want of a better term). Mounted on the aircraft belly, it is intended to receive straight down as the aircraft overflies the Marker station.

The Marker station consists of a low power transmitter , around 6W for an Outer Marker, and a 3el yagi pointed straight up.

It's not a DX antenna, the range is only around 3000ft for a light aircraft on Approach.

I would suggest that such an antenna for HF would be probably ideal for NVIS, but little else.