[TowerTalk] newbie question on inverted-L's, tuners, and radials
n6ry at arrl.net
Wed Jun 21 21:01:59 EDT 2006
At 11:40 AM 2006-06-21, Eugene Hertz wrote:
>I am attempting to erect an inverted-L antenna for my shack mostly
>because of trees/mounting points (making a little difficult for a
>dipole or similar). It will be the only antenna that I have (my
>first shack!). The planned arrangement is as follows:
>Feedline comes from my second floor shack down to the ground. I
>will (hopefully) use a remote antenna coupler, a Harris
>RF-601A. This tuner will be grounded using radials (see next
>paragraph). From the tuner, a single wire will emanate up the side
>of the house to a mast on my chimney. This should get the height of
>the vertical portion of the antenna about 40' up. The wire will then
>traverse across my front lawn to a tree about 140' away and still
>about 40' up. I would love to be able to get some signal out on 10-160.
As others have mentioned, the patterns on 80m and up will be quite
"interesting". On 80m it will perform much like a low dipole, with
some nulling off the ends of the horizontal wire and lots of high
angle radiation. On the higher bands, it works more like a long
wire, with the tendency for the highest gain lobes to fall towards
the ends of the wire. Certainly, you will be able to work lots of
stations, but you will find that signals may not always be optimal in
the direction you might desire.
With the vertical section very close to your house, you may
experience TVI/RFI problems, depending on your transmitter power
output. You may also hear the unwanted RF junk from every electronic
device in your home on receive, too.
>The Harris tuner is designed for shipboard and shore installations
>and can match 50 ohms to a whip of 15'-35' from 2-30Mhz. Now, I am
>not sure what this implies as far as range of impedence matching
>capabilities. But perhaps something can be derived from the height
>of the whip, probably using the ship itself as the ground plane?
>More information can be found here:
The RF601A has a wideband step-down transformer (50 ohms to 22 ohms)
followed by a high-pass step-up L-network. It might work OK on all
bands, but that really depends on the range of values of the variable
L and C in the autotuner. In most tuners, the lowest bands have the
most restricted impedance matching range, but fortunately your
selected wire length does provide relatively benign
impedances. However, your inverted-L will present a very different Z
than a 35 foot whip, especially on 160 and 80m. Unfortunately, this
is one of those situations that requires experimentation.
>Here come the questions:
>1. Most of the discussion I read about radials had to do with
>"vertical" antennas. I am wondering if all those discussions could
>be more generalized to pertain to all end-fed antennas such as the
>"L"? Is there anything different about the use of radials (sizes,
>locations, numbers) as applied to L's vs verticals?
A radial system designed for a vertical antenna should work well for
>2. I have an issue with the possible location of my radials. The
>vertical part of the antenna is very close to my house, therefore My
>radials can be 180deg _away_ from the direction of the horizontal
>wire, as the wire travels over the house (not exactly, but you get
>the idea). In fact, I may only have some short radials 180 deg away
>from the antenna, but can do longer ones +/- 90 deg from the
>horizontal wire. Question is, with a long horizontal component, is
>there any necessity to have the radials "beneath" the horizontal
>portion? For example, if I can get 30 radials in the ground but none
>of them are beneath the antenna is this terribly worse than 30 in
>the ground where some are beneath the antenna?
I did some quick EZNEC models with/without radial wires under the
horizontal section. There is some difference, but the results are
somewhat inconsistent (partly because I only have the standard NEC2
version, and the accuracy with radials very near ground is
questionable). In general, it will probably work OK even if you
don't have radials directly under the 140 foot wire. You will
probably want to connect at least a couple of radials from the base
of the antenna that bend around the foundation of the house. Making
any radial longer than 1/4 wavelength overall (~125 feet) on 160m is
probably a waste of wire.
>3. With the heights and lengths described above, should I think
>about shortening/lengthening the wire (perhaps to get onto 160?) Or
>should I let the tuner do the work and just make the longest wire
>possible? Bands of interest are primarily 160,80,40,20,15,10 (until
>I get my urt-23 running then its all ham bands).
The total wire length (40+140=180 feet) is about 1/3 wavelength on
160m, which is a good compromise that avoids really high feed
impedances on most bands. It also gives a reasonable feed Z on 160m
that should provide a little higher efficiency (less ground loss)
than a 1/4 wavelength (or shorter) vertical or inverted-L.
PS It's interesting that a guy named Hertz is planning to use a
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