Inverted L as 1/2 wave
Peter J. Bertini
I've been playing with Reg's DOS programs trying to come with a low band
antenna for 160/80 and 40 meters. So far I've come up with a 40-meter
dipole feed with 1/4 wave of 450-ohm ladder line. I also intend to feed the
antenna in a T configuration, running as a 1/4 wave end fed on 160, and a
1/2 wave on 80 meters.
I assume the 1/2 wave gives a relatively high DX angle, vs. what a 1/4 wave
would provide. But, how do things 'even out' in the real world given the
extremely high efficiency of the halfwave due to less dependence on the
radial system? I would have a hard time installing a radial system for a
1/4 wave that would come close to the efficiencies with a 1/2 wave radiator.
Any ideas how I could squeeze a 30 meter dipole into this mess?
Pete
--
Brian Kelly, w3rv
<comm...@home.com> wrote:
>Pardon the elementary nature of this post..
>
>I've been playing with Reg's DOS programs
>
He programs with Pasca. They do run nicely in DOS windows.
>
> trying to come with a low band
>antenna for 160/80 and 40 meters. So far I've come up with a 40-meter
>dipole feed with 1/4 wave of 450-ohm ladder line. I also intend to feed the
>antenna in a T configuration, running as a 1/4 wave end fed on 160, and a
>1/2 wave on 80 meters.
>
I've been trying to sketch your antenna in my head and I'm completely
confused. Check me: You have a 40m dipole (70 feet long) fed in the
center with a quarter wave length of ladder line (35 feet)?? That
would be a total of 3/4 wavelength worth of wire on 40m, 3/8 of a
wavelength of wire on 80m and 0.188 wavelength on 160. I doan get it.
>
>I assume the 1/2 wave gives a relatively high DX angle, vs. what a 1/4 wave
>would provide. But, how do things 'even out' in the real world given the
>extremely high efficiency of the halfwave due to less dependence on the
>radial system? I would have a hard time installing a radial system for a
>1/4 wave that would come close to the efficiencies with a 1/2 wave radiator.
>
An inverted L is usually considered a half-wave end fed wire half of
which is vertical and half of which is horizontal on it's lowest
frequency of operation. The feedpoint impedances are huge which
minimizes the need for radials. I've had 80m inverted Ls and I can
attest to the fact that they work really well on both 80 and 40. 65
feet up vetically and 65 feet horizontally with a simple ground rod at
the tuner. But they were terrible antennas on 160. A 160m inverted L
would require 270 feet of wire to be a decent antenna without radials.
???
>
>Any ideas how I could squeeze a 30 meter dipole into this mess?
>
>
>Pete
>
w3rv
Peter J. Bertini
"Brian Kelly, w3rv" <ke...@nospam.dvol.com> wrote in message
news:3acfe581...@news2.dvol.com...
> On Sat, 07 Apr 2001 04:10:16 GMT, "Peter J. Bertini"
> <comm...@home.com> wrote:
>
> >
> I've been trying to sketch your antenna in my head and I'm completely
> confused. Check me: You have a 40m dipole (70 feet long) fed in the
> center with a quarter wave length of ladder line (35 feet
Tower is 65 feet, so feedline would be nearly 1/4 wave on forty; which
would really only work if I had enough height for 1/2 wave of ladder line.
Looks it will have to be coaxial feed.
>
> >
>> >
> An inverted L is usually considered a half-wave end fed wire half of
> which is vertical and half of which is horizontal on it's lowest
> frequency of operation.
I thought the most common inverted Ls used 1/4 wavelengths; wouldn't 1/2
wave place the high current point at the transistion point between
horizontal and vertical polarization? Or, would that have some benefits for
short range communications?
The feedpoint impedances are huge which
> minimizes the need for radials. I've had 80m inverted Ls and I can
> attest to the fact that they work really well on both 80 and 40. 65
> feet up vetically and 65 feet horizontally with a simple ground rod at
> the tuner. But they were terrible antennas on 160. A 160m inverted L
> would require 270 feet of wire to be a decent antenna without radials.
> ???
> >
The plan was a 1/4 wave T on 160, coax feed dipole for 40, and 1/2 wave T
on 80, all operating against the best radial system I could manage, which
would be marginal.
> >
> >
> >Pete
> >
> w3rv
KC1DI
"Peter J. Bertini" <comm...@home.com> wrote in message
news:GH0A6.93700$Ok4.7...@news1.rdc1.ct.home.com...
effect here over the years.. though a Half wave wire does not require alot
of radials , they do help with ground loss even on a 1/2 wave wire. My
favorite inverted L for the low bands is a 3/8 wave for 160M about 170 feet
long , and with tuner it works great on 160 , 80, and pretty fair on 40 m
also .
73 dave kc1di
---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.247 / Virus Database: 120 - Release Date: 4/6/01
Brian Kelly, w3rv
<comm...@home.com> wrote:
>Tower is 65 feet, so feedline would be nearly 1/4 wave on forty; which
>would really only work if I had enough height for 1/2 wave of ladder line.
>Looks it will have to be coaxial feed.
>
Now I see where the problem is. You have some numbers twisted around a
bit Pete. A 1/4 wavelength on 40 is 33 feet, not 65 feet. Your tower
is about 1/2 wavelength tall on 40.
>>
>
>I thought the most common inverted Ls used 1/4 wavelengths; wouldn't 1/2
>wave place the high current point at the transistion point between
>horizontal and vertical polarization?
>
Yes, absolutely. In the case of an 80m L on your tower the vertical
portion would a 1/4 wavelength long at the 65 foot point then another
65 feet horizontally to another support.
>
And yes, the current max would be at the transition point. Which is
exactly what you want. The current max at the highest point in the
system is one of the beauties of an L. The last place you want the
current max is on the ground at the tuner. Even with a radial farm.
Plus with an L you get some vertical polarization.
>
It gets even better on 40. On 40 the 65 foot vertical portion becomes
a halfwave vertical which is a 3dB advantage over a 1/4 GP with
radials. Plus the horizontal portion makes it's contribution as
another half wave radiator.
>
There are obviously pattern issues involved and after I finally quit
talking about it and bite the wallet for the software one of my first
modeling exercises will be studying Ls.
> Or, would that have some benefits for
>short range communications?
>
My experience has been quite the opposite. I could barely grab one JA
multiplier on 40 with an elevated GP (base at 35 feet) on 40 in the
dx contests and I never did work a JA on 80 with my 135 foot open-wire
fed inverted vee from here on the east coast. Got the first 80m L
running and I ran JAs on both bands. Dramatic improvement on the
long-haul stuff.
>
>The plan was a 1/4 wave T on 160, coax feed dipole for 40, and 1/2 wave T
>on 80, all operating against the best radial system I could manage, which
>would be marginal.
>
A lot of it depends on how much real estate you have available. I
think you would find that an 80/40 L with the vertical run spaced
maybe 30 feet away from the tower would be a better antenna than the
dipole/T combination.
>
160 is a pain in the butt, you probably would be better off using a T
vs. the 80/40 L at least from the standpoint of ground losses. A 160
tee in your case would be 65 feet up vertically to the center of a 205
foot flattop. If you have the space I think you'd be better off with
an end-fed 1/2 wire, 65 feet up and 205 feet horizontal, a distorted L
configuration.
>
>> >Pete
>
w3rv