Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: 160M Tee Antenna - Design & Theory Question  (Read 5427 times)
K3VAT
Member

Posts: 756




Ignore
« on: November 02, 2012, 07:47:01 PM »

Given: A 160M Tee antenna with 60 vertical feet of #14 and two ~ 45 horizontal sections (same wire), having a fair/good radial field (24 quarter-wave ground radials) and with no obstructions within ½ wavelength.

What exactly happens to the E and M fields and other RF parameters when the upward accelerating current hits the horizontal wire?  Stated another way, what are the effects of 90 degree bend in the conductor have on key antenna parameters such as feed point impedance, efficiency, field strength and radiation pattern?

Would utilizing much large conductors (such as 1” aluminum tubing) at the top of the vertical section and for the first, say, 6 to 8 feet of the horizontal section have any positive, meaningful result?

73, Rich, K3VAT

Logged
WX7G
Member

Posts: 6210




Ignore
« Reply #1 on: November 03, 2012, 07:28:50 AM »

The top structure has current flowing in opposite directions. So, the magnetic field from one 45' wire largely cancels the magnetic field of the other side and the result is virtually no radiation. The 60' vertical wire does all of the radiating. The horizontal wire top loads the antenna and changes the current distribution along the vertical wire. With the horizontal wire and 1 amp at the base the current is 0.77 amps at the top. Without the horizontal wire the current at the top is zero. With the horizontal wire the "current-area" of the antenna is increased resulting in the higher radiation resistance of 19 ohms vs 7 ohms.

Using thicker wires in the horizontal portion will lower the resonant frequency. A NEC model shows your antenna being resonant at 1.905 MHz.

This antenna can be modeled with the free version of EZNEC. That version is limited to 20 current segments so one would use 8 segments for the vertical wire and 6 for each of the horizontal wires.

The radials need not be 1/4 wavelength. 1/8 wavelength radials will work just as well. See the N6LF radial papers for this analysis. I have a 29' base loaded vertical with 90 radials from 12 to 24 ft. On 160 meters the ground loss is about 5 ohms.
« Last Edit: November 03, 2012, 07:40:26 AM by WX7G » Logged
K3VAT
Member

Posts: 756




Ignore
« Reply #2 on: November 03, 2012, 10:06:37 AM »

...
Using thicker wires in the horizontal portion will lower the resonant frequency.
... 

Thanks Dave,  So why does using thicker wires in the horizontal portion lower the resonant freq?  I would have thought that perhaps there would be a corresponding increase in bandwidth (albeit small).
Logged
WB6BYU
Member

Posts: 13482




Ignore
« Reply #3 on: November 03, 2012, 10:39:33 AM »

If you want wider bandwidth, use two vertical wires connected 10' or so apart
to the top wire.  That makes a very fat lower radiator:  my model suggests that
10' spacing doubles the SWR bandwidth at 2 : 1.  You can feed the two wires
in parallel at ground level, or ground one and feed the other to step up the
feedpoint impedance.

I used a similar system on 80m and covered at least 400kHz at usable SWR.
The only addition is the second vertical wire.
Logged
K3VAT
Member

Posts: 756




Ignore
« Reply #4 on: November 03, 2012, 11:47:33 AM »

...
I used a similar system on 80m and covered at least 400kHz at usable SWR.
The only addition is the second vertical wire.

Thanks Dale.  So for your 80M vertical what is the vertical height portion and the horizontal length?  Is it a tee configuration or perhaps you're just using a capacitive tophat?  And what does the top (horizontal) portion look like?  Thanks!



Logged
WB6BYU
Member

Posts: 13482




Ignore
« Reply #5 on: November 03, 2012, 12:54:14 PM »

This was over 30 years ago, so I'm going from memory.

I had a wood mast perhaps 40' tall.  The top wire was about 50', with the
center hung from the top pulley and the ends sloping down perhaps 30 degrees
from horizontal.

Out about 4 - 6' or so on each side of the center I connected the vertical
wires, which came together at the feedpoint right above ground level.
I used a couple ropes at some point to pull the ropes out to the sides
to keep the radiator fat.

A quick model with 6' spacing between vertical wires suggests 200kHz SWR
bandwidth (relative to 18 ohms) on 80m with no ground losses.  Since my
ground system was less than optimum, the SWR was pretty good at resonance,
which suggests 20+ ohms of ground loss resistance, making it fairly easy to cover
the whole band.

I built it well before computer modeling was common, and just decided to
make a fat vertical to improve bandwidth.  Dimensions were dictated by
the height of my mast and where I could tie the ends off - there isn't
any particular magic to it.

It looks like this approach requires a few more feet of top wire to resonate
at the same frequency as a conventional T antenna - a good starting point
would be to assume that the length of the top wire between the vertical
wires needs to be made up for on the ends.
Logged
K3VAT
Member

Posts: 756




Ignore
« Reply #6 on: November 04, 2012, 05:07:30 AM »

...
A quick model with 6' spacing between vertical wires suggests 200kHz SWR
bandwidth (relative to 18 ohms) on 80m with no ground losses.  Since my
ground system was less than optimum, the SWR was pretty good at resonance,
which suggests 20+ ohms of ground loss resistance, making it fairly easy to cover
the whole band.
...

Thanks Dale,  200 kHz SWR B/W is decent for that arrangement.
If you have the time, please do a model for me based upon the following:
Vertical Section = 52 feet using two #14 wires connected at the base and diverging upwards to attach with a spread of 6 feet.  Horiz section = Tee arrangements with equal wire (#14) length on either side with flat top for 16 feet (using 1.5" aluminum tubing; 8' per side) then sloping downward at 45 degs for the remaining length using #14 wire.  Desired F
  • = 1.8mHz; soil condx (good to very good); radials = twenty-four 65 footers evenly spaced.  I estimate ground losses here at between 10 and 15 ohms for this antenna.

Appreciate it!  73, Rich, K3VAT
Logged
WB6BYU
Member

Posts: 13482




Ignore
« Reply #7 on: November 04, 2012, 08:09:28 AM »

Quote from: K3VAT

Vertical Section = 52 feet using two #14 wires connected at the base and diverging upwards to attach with a spread of 6 feet.  Horiz section = Tee arrangements with equal wire (#14) length on either side with flat top for 16 feet (using 1.5" aluminum tubing; 8' per side) then sloping downward at 45 degs for the remaining length using #14 wire.  Desired F
  • = 1.8mHz...

Not a good approach, actually, because the end wires reach nearly to ground level
by the time the resonant frequency reaches 1.8 MHz, which lowers efficiency (besides
being a safety hazard.)  Radiation resistance of the antenna is about 5 ohms.

A general rule is that sloping wires used for top loading shouldn't extend much more
than half way down the antenna to maintain efficiency.  (Actually I think the number
from the study I read was 40% of the antenna height, but half way is easier to
remember.)

So instead I fixed the end height of the wires at 25' and varied the length, letting the
angle be whatever it is.  The required that the wires extend out about 70' from the
center on each side (68' of wire on each end of the center crossbar.)  Radiation
resistance is 7.5 ohms:  when matched to that value, the SWR bandwidth is about
40 kHz at 2 : 1 (though part of that is below the bottom of the band.)

Adding 15 ohms of ground loss resistance and matching the antenna to 22 ohms
gives a 2 : 1 SWR bandwidth from 1.75 to 1.87 MHz, or 120kHz.

This assumes that the vertical wires maintain equal spacing most of the way down
to the feedpoint.  If I bring the bottom ends closer together (there are some
modeling issues with bringing wires together at small angles) the passband lowers
by about 20 kHz.  Shortening the loading wires to 64' on either end of the
aluminum tubing puts the 2 : 1 passband from 1.79 MHz to 1.9 MHz, with an
SWR of 1.5 : 1 at 1.8 MHz.

One observation is that there is no current flowing in the center of the aluminum
tubing between the attachment points of the vertical wires.  That means that using
an insulator that point would allow such an antenna to serve as a doublet for 80m
and higher bands simply by changing the configuration at the feedpoint.
Logged
K3VAT
Member

Posts: 756




Ignore
« Reply #8 on: November 04, 2012, 12:55:21 PM »

Quote from: K3VAT

Vertical Section = 52 feet using two #14 wires connected at the base and diverging upwards to attach with a spread of 6 feet.  Horiz section = Tee arrangements with equal wire (#14) length on either side with flat top for 16 feet (using 1.5" aluminum tubing; 8' per side) then sloping downward at 45 degs for the remaining length using #14 wire.  Desired F
  • = 1.8mHz...
...
Not a good approach, actually, because the end wires reach nearly to ground level
by the time the resonant frequency reaches 1.8 MHz,

Ops!  You're correct.  Wires will come off the flat top ~45 degs, but will tie off at ~30 feet high in distant trees so the shape of the wire curve would be a catenary.

...
One observation is that there is no current flowing in the center of the aluminum
tubing between the attachment points of the vertical wires.  That means that using
an insulator that point would allow such an antenna to serve as a doublet for 80m
and higher bands simply by changing the configuration at the feedpoint.

I did not know that - interesting observation.  So one could therefore use a 6' piece of fiberglass tubing at the center as long as the vertical attachment points were fixed at > 6'.

... I fixed the end height of the wires at 25' and varied the length, letting the
angle be whatever it is.  The required that the wires extend out about 70' from the
center on each side (68' of wire on each end of the center crossbar.)  Radiation
resistance is 7.5 ohms:  when matched to that value, the SWR bandwidth is about
40 kHz at 2 : 1 (though part of that is below the bottom of the band.)  Adding 15 ohms
of ground loss resistance and matching the antenna to 22 ohms gives a 2 : 1 SWR
bandwidth from 1.75 to 1.87 MHz, or 120kHz. 
This assumes that the vertical wires maintain equal spacing most of the way down to
the feedpoint.  If I bring the bottom ends closer together (there are some
modeling issues with bringing wires together at small angles) the passband lowers
by about 20 kHz.  Shortening the loading wires to 64' on either end of the
aluminum tubing puts the 2 : 1 passband from 1.79 MHz to 1.9 MHz, with an
SWR of 1.5 : 1 at 1.8 MHz.
...

These are very promising figures Dale, thanks a lot!  73, Rich, K3VAT
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!