Antennas with resistor and OCFD.

classic Classic list List threaded Threaded
5 messages Options
Reply | Threaded
Open this post in threaded view
|

Antennas with resistor and OCFD.

Brian D. Comer
I think stating any particular antenna configuration is unacceptable is
ignoring what could be an ideal antenna for a given situation.  As I
understand  it  a rhombic  uses a resistor, and sometimes works quite well.
On 160 I have an OCFD 1/2 wavelength that is goes from one side of the end
of a very small canyon  to the other.  The net result is that the center of
the antenna is about 80 feet above ground, the ends are only 50 ft above
ground. On flat ground this would be a NVIS antenna being so low; however,
with the ground 100 feet to the west at the same height as the antenna and
the ground 100 feet to the east being 300 feet below the antenna  I suspect
the pattern looks more like a beam pointing east. Considering I have broken
two important rules, the antenna is OCFD, and is on average it is well under
1/2 wavelength above ground it is amazing I can work the east so well.

 

73 Brian KF6C

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]
Reply | Threaded
Open this post in threaded view
|

Re: Antennas with resistor and OCFD.

Barry K3NDM
Brian,
      Be careful of your logic. The reason a rhombic works so well is
that most rhombics are 2 or more wavelengths long per leg. However,
putting in a resister makes the beast directional toward the resister,
not all bad nor inefficient. But the T2FD type antenna is only a 1/2
wave long on the lowest frequency making it more efficient as you go up
toward 10 meters.

     I have been reading a lot of comments on antennas here, some are
correct and some just may be close at best. Let me reiterate something
I've said earlier and regularly. There is NO perfect antenna. Every one
is some compromise in one way or another. in my case, I need to deal
with 40 meter space with a desire to use 80 meters. So, part of my
horizontal antenna is bent down toward the ground. It fits. Do I have
the best antenna?? No. But, it does fit.

     OCFD antennas are not really bad antennas if you follow the rules of
horizontal antennas. If the antenna was fed in the center, it would be
50 Ohms or nearly so at its 1/2 wave length. Above that, even multiples
of the 1/2 wave frequency, the feed impedance would be very high. So, it
was figured out that you can feed off center and could find a spot that
would not be awful at the even harmonics. A 4:1 current balun will take
of most of the impedance issue and bring it down into range of your
tuner. A good balun will have a loss of less than 1 db. If you use good,
low loss coax, losses will not be bad.

     The 1/2 wave dipole, center fed dipole. fed with open wire and a
balun at the shack entrance makes a good antenna, and the losses may be
a slight amount lower than the OCFD, but it follows the rules of
horizontal antennas that are a specific height.

     There is nothing in ham radio that is argued more passionately than
antennas, with maybe the differences between Windows and the Apple OS.

73,
Barry
K3NDM

------ Original Message ------
From: "Brian D. Comer" <[hidden email]>
To: [hidden email]
Sent: 12/10/2015 7:14:24 PM
Subject: [Elecraft] Antennas with resistor and OCFD.

>I think stating any particular antenna configuration is unacceptable is
>ignoring what could be an ideal antenna for a given situation.  As I
>understand  it  a rhombic  uses a resistor, and sometimes works quite
>well.
>On 160 I have an OCFD 1/2 wavelength that is goes from one side of the
>end
>of a very small canyon  to the other.  The net result is that the
>center of
>the antenna is about 80 feet above ground, the ends are only 50 ft
>above
>ground. On flat ground this would be a NVIS antenna being so low;
>however,
>with the ground 100 feet to the west at the same height as the antenna
>and
>the ground 100 feet to the east being 300 feet below the antenna  I
>suspect
>the pattern looks more like a beam pointing east. Considering I have
>broken
>two important rules, the antenna is OCFD, and is on average it is well
>under
>1/2 wavelength above ground it is amazing I can work the east so well.
>
>
>
>73 Brian KF6C
>
>______________________________________________________________
>Elecraft mailing list
>Home: http://mailman.qth.net/mailman/listinfo/elecraft
>Help: http://mailman.qth.net/mmfaq.htm
>Post: mailto:[hidden email]
>
>This list hosted by: http://www.qsl.net
>Please help support this email list: http://www.qsl.net/donate.html
>Message delivered to [hidden email]

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]
Reply | Threaded
Open this post in threaded view
|

Re: Antennas with resistor and OCFD.

k6dgw
In reply to this post by Brian D. Comer

On 12/10/2015 4:14 PM, Brian D. Comer wrote:
> I think stating any particular antenna configuration is unacceptable is
> ignoring what could be an ideal antenna for a given situation.

Absolutely!  Situations differ dramatically.  A low angle radiator on 80
in the evening is likely to underwhelm you if your need is to
communicate over 150 miles.

>  As I
> understand  it  a rhombic  uses a resistor, and sometimes works quite well.

Ummm ... rhombics *always* work well for their intended use.
Unterminated, they are bi-directional.  With the terminating resistor,
they are unidirectional in the direction from the feedline to the
terminator.  Since terminated rhombics are usually 2 or more wavelengths
on a side, they have very narrow beamwidths with a big F/B ratio, low
radiation angle, and high gain.

They were used on point-to-point HF circuits in the 30's through maybe
the 60's or so.  I worked coastal marine in 56-57.  We generally used
V-beams with wider beamwidths since our targets were ships that moved
around, but we had a couple of rhombics, one aimed at NMO in Hawaii.
Tap the key at 5 KW and they told us we were QSA 11R5:-)

OCF's can serve well in the right situations and with the right
precautions to mitigate the downsides.  The infamous B&W folded dipole
was never billed as anything other than what it was ... an antenna with
a non-inductive resistance that pretty much swamped any effects from the
antenna, and could be used 3 - 30 MHz, in the right situations.

Tom Schiller, N6BT, likes to show his "illuminator", a 300 W light bulb
fed through a common mode choke.  He even put three up as a "phased
array.  Any RF in any conductor will radiate.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the Cal QSO Party 1-2 Oct 2016
- www.cqp.org

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]
Reply | Threaded
Open this post in threaded view
|

Re: Antennas with resistor and OCFD.

Jim Bolit





The Rake



-------- Original message --------
From: Fred Jensen <[hidden email]>
Date: 12/10/2015 5:54 PM (GMT-10:00)
To: [hidden email]
Subject: Re: [Elecraft] Antennas with resistor and OCFD.


On 12/10/2015 4:14 PM, Brian D. Comer wrote:
> I think stating any particular antenna configuration is unacceptable is
> ignoring what could be an ideal antenna for a given situation.

Absolutely!  Situations differ dramatically.  A low angle radiator on 80
in the evening is likely to underwhelm you if your need is to
communicate over 150 miles.

>  As I
> understand  it  a rhombic  uses a resistor, and sometimes works quite well.

Ummm ... rhombics *always* work well for their intended use.
Unterminated, they are bi-directional.  With the terminating resistor,
they are unidirectional in the direction from the feedline to the
terminator.  Since terminated rhombics are usually 2 or more wavelengths
on a side, they have very narrow beamwidths with a big F/B ratio, low
radiation angle, and high gain.

They were used on point-to-point HF circuits in the 30's through maybe
the 60's or so.  I worked coastal marine in 56-57.  We generally used
V-beams with wider beamwidths since our targets were ships that moved
around, but we had a couple of rhombics, one aimed at NMO in Hawaii.
Tap the key at 5 KW and they told us we were QSA 11R5:-)

OCF's can serve well in the right situations and with the right
precautions to mitigate the downsides.  The infamous B&W folded dipole
was never billed as anything other than what it was ... an antenna with
a non-inductive resistance that pretty much swamped any effects from the
antenna, and could be used 3 - 30 MHz, in the right situations.

Tom Schiller, N6BT, likes to show his "illuminator", a 300 W light bulb
fed through a common mode choke.  He even put three up as a "phased
array.  Any RF in any conductor will radiate.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the Cal QSO Party 1-2 Oct 2016
- www.cqp.org<http://www.cqp.org>

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]
Reply | Threaded
Open this post in threaded view
|

Re: Antennas with resistor and OCFD.

Fred Townsend-2
In reply to this post by k6dgw
True rhombics are hard to rotate. Even harder is finding the space to put
one up in the first place. See what your HOA has to say about it.
73
Fred, AE6QL

-----Original Message-----
From: Elecraft [mailto:[hidden email]] On Behalf Of Ron
D'Eau Claire
Sent: Thursday, December 10, 2015 8:43 PM
To: [hidden email]
Subject: Re: [Elecraft] Antennas with resistor and OCFD.

Fred, K6DGW wrote:

Ummm ... rhombics *always* work well for their intended use.
Unterminated, they are bi-directional.  With the terminating resistor, they
are unidirectional in the direction from the feedline to the terminator.
Since terminated rhombics are usually 2 or more wavelengths on a side, they
have very narrow beamwidths with a big F/B ratio, low radiation angle, and
high gain.
------------------------

Quite so but it's worth noting that the unidirectional characteristics
obtained with a terminating resistor did not result in any additional gain.
Instead, the resistor absorbed the RF that would have been radiated on (or
received from) the reciprocal heading, giving the array its unidirectional
characteristic.

Yagi's, quads and similar arrays gain a stronger main lobe by suppressing
radiation in other directions.  

A rhombic is a classic example of how the best design is not always the
design that radiates the most RF.
The problem with a rhombic for Ham use on the HF bands is that they tend to
be an absolute beast to try to rotate.

73, Ron AC7AC

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html Message
delivered to [hidden email]

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]