Hi Gabriel,
There is too many variables in your antenna setup to determine the expected
impedance.
I have built and measured several inverted L's with/without elevated radials
and/or sloping top wires.
So I will refrain from putting out estimations, other than guesses from what I
learned during the last 30yrs.
What I do know:
Fact: an inverted L with 1/8 vertical and 1/8 straight horizontal offers a
radiation resistance Rs=18 Ohm. No more.
Fact: Antenna impedance (Z)= Rs+Rg. Rground refers to energy not radiated
but lost in the radials to ground.
Your Antenna Analyzer/VNA will offer at resonance (find the frequency at which
Xc=0) is Z. You now have two unknown variables(Rs and Rg) and one measured
value Z
Fact: Z, SWR_usable_Bandwidth, Rground, Rs and any device to match it all to 50
Ohm are variables which can be a challenge. But don't sweat it.
Fact: making the vertical part higher v.s. sloping wire=> get the sloping wire
up. That is much more effective.
My experience:
- A straight horizontal L-part is just to about impossible; at 1/8 length any
wire will droop in the middle and lower Rs substantially. Spent two days
measuring just that when I had two 23m HD tubular towers.
- A sloping single top wire quickly lowers the Rs. In your setup I would
expect it to be between 11 and 13 Ohm
- knowing that guestimated value; when your analyzer shows Z=40 Ohm SWR=great
at resonance (Xc=0) one can calculate the ground losses Rg=40-12=28Ohm. That
will be with 4 radials on ground probably. A nice SWR but effectivity is
12/40= 30% the other 28/40-70% of the power goes into ground.
- Good SWR does not mean the antenna is 'really good'. But don't seat it,
sometimes you have limited options. And you have to start somewhere.
- Elevated radials: been there. 4 resonant elevated radials is possible, but
difficult to measure correctly such that all four behave similar. We had them
at 4m height. Higher is better, more is better.
- If you lengthen the L-part so the total becomes 50 to 54m and tune out that
extra length using a series Cap at the feedpoint, you can raise the Z. We did
it with our 4 elevated radials and got a Z=34 Ohms which was nice to use a
W2FMI 32:50 Ohm transformer.
- Radials; put down about 30-50pcs with 25 to max 30m length. Want to lower
Rground more effectively? Go to 120 radials at longer lengths, OR
- Additionally put lengths of chicken wire directly under the vertical. Do not
try to make length in a start config like radials; instead just closely cover
straight lengths, make a square. It does not have to make electrical contact
(it will help a bit but not the effort worth). Been there, measured it.
- At a good setup you probably will have a Z= 12+15=27 Ohms (there it is I did
make an estimation!) my point here:
- You will need a matching device / transformer (one toroid <500W, two or three
at 1K5) to get from the guestimation value to connect it to your 50 Ohms coax
feeder into the shack.
- There will be many other voices from other people having
less/similar/more/different experience.
My tips:
- Don't sweat it. At 1/8 vertical part, even with a sloping wire, one can make
a good signal. Even at some crooked elevated radials.
- Read Low Band Dx'ing by ON4UN; it tells you how to do it, both in KISS mode
as well as freaking nerd-mode. There will not be a great difference unless you
have the hardware AND lots of real estate.
- Use an antenna analyzer which shows Xc, Z and SWR; the cheapest Rig Expert or
any like is best in the field (no MFJ). I have had several different ones in
the past 20yrs.
- Use a portable VNA if you must, but only if you have the experience how to
use it. Or have someone come over to do that 😊. No Smith chart crap; that's
for nerds and NOT effective in the field, unless you use smith charts on a
daily basis.
- Measure at the antenna, using a pigtail coax to about 0,5 to 1m length. Yes
one can measure it in the shack using professional network analyzers/ VNA's
etc. when the feedline is properly calibrated out. Been there. A friend has a
large HP in his shack. In the end we prefer that small but very practical Rig
Expert.
Happy experimenting!
73
Mark, PA5MW
-----Original Message-----
From: Topband <topband-bounces+pa5mw=home.nl@contesting.com> On Behalf Of
Gabriel - EA6VQ via Topband
Sent: Thursday, March 05, 2020 19:02 PM
To: topband@contesting.com
Subject: Topband: Inverted L with elevated radials for topband
I have some doubts about installing and inverted L with elevated radials for
160m. I have been searching in Google and find some contradictory information,
so I would appreciate very much if you can help me with your own experience.
The antenna would be supported by a 16 m (52.5 ft) high fiberglass pole placed
on top of a 3m (10 ft) high small tower with the horizontal part of the L
slopping down to a 5 m (16 ft) high mast about 23 m (75 ft) away. Two to four
tuned elevated radials can be placed, although they will have to be bent due to
space restrictions.
Now the doubts:
What impedance can I expect at the feed point with this configuration?
How to match it in order to feed it with a 50 ohm cable? Some pages say that
no match is required, only a choke. Other pages say a hairpin is necessary as
the impedance can be too low. So other say that a tuner at the
base is required? I am confused L
What performance for DX can be achieved by this antenna? Is it really good or
do you have some better suggestion for a really small lot where no ground
radials are possible?
Would it be worth to use a higher fiberglass pole, let's say 4 m (13 ft)
higher, in order to lengthen the vertical section of the L? Would the
difference be noticeable?
Anyone has real experience setting up this antenna with elevated radials?
Most information I can find on Internet is related to ground mounted antennas.
Thanks in advance!
73. Gabriel - EA6VQ
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