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 "The >L<
Network Revisited" by
N3AAZ
Originally published in the Nor-Cal QRPp newsletter.
Special thanks to QRPp and it's editor Doug (KI6DS) for inspiring John (N3AAZ)
to write this article.
What do these terms have in common?
QRP, backpacking, efficiency, zero-weight feedline, low pass filter, stealth, home brew,
omni-directional, all band, and tuner.
Answer... One inductor and one capacitor, the >L< Network.
Figure 1 shows the no feedline, portable, long wire antenna system I used with much
success a few years ago while camping in a pop-up. This setup will match most 100 foot long wire antenna configurations form 80 through 10 meters.
Since the components are small, light, easy to pack and deploy this configuration also makes for an outstanding spare or emergency
configuration.
The inductor is 21 turns of solid 18 AWG wound on a "stack" of three,
Micrometals, RED (MIX 2), T94-2, powdered iron cores, with a "tap" placed every three turns
(Figure 3). The capacitor is a 200 pico Farad variable.
The antenna wire is a small roll of inexpensive, insulated, zip cord. You will need "alligator" clips and a tuning indicator. The earth ground
system is several strands of zip cord in parallel. The ground rod just a common screwdriver with a vice-grip for hammer and wire clamp.
Feedline is not required (in Figure 1) because the >L< network is
connected direct between transceiver and base (the feed point) of the antenna. The feedpoint of the long wire antenna is located inside the
shack (camper, tent or sleeping bag).
The tuning indicator can take many forms. It is noteworthy to say, if it takes too long to adjust ANY antenna tuner, the "final RF amplifier" in
your transmitter may be at risk to over heating. I suggest, install a 10 dB attenuator
(Figure 4) between the transmitter and tuner to help
protect the PA transistor during the initial adjustment. The "attenuator" technique is NOT a guarantee. The safest method is reduced transmitter
power and short (intermittent) "key down" adjustments. The attenuator is removed for final adjustment and QSOs.
The >L< Network
For the purpose of this discussion the term long wire antenna refers to a non-resonant wire element that is either, straight line, zigzag,
horizontal, vertical, or any combination, but always longer than a half wave at it's operating frequency. A counterpoise is not required. An
earth ground helps.
The approximate half wave length in feet can be determined by dividing 468 by the operating frequency in MHz. Example, Length(ft) = 468 /
F(MHz), a half wave at 7.04 MHz is approximately 66.5 feet.
How does the >L< network match a low impedance transmitter to high impedance long wire? Connect the coil in series between the XCVR / ANT
junction and connect the capacitor from the COIL / ANT junction to the XCVR chassis and earth ground, see
Figure 1.
The >L< network will also match most random, non resonant, "short", low impedance antenna. The short antenna refers to a wire or rod (whip),
straight line, zigzag, vertical, horizontal or any combination but always less than a quarter wave at it's operating frequency. The short antenna
is a compromise, efficiency can be poor and a counterpoise is required for best results. When matching a highZ (50 Ohm, transmitter) to lowZ
(15 Ohm, short vertical) the inductor is connected in series between source and load. The capacitor is "shunt" or parallel to the source, see
Figure 2. Note, this connection is unlike the "long" example above.
Today our camper is a fifth-wheel with an eight-foot aluminum ladder rack. I just add a Radio Shack # 21-937B Rack Mount Bracket to create a
very nice whip antenna support AND ladder rack counterpoise antenna system. The base of my antenna is now atop that ladder rack so a feedline
is required. Feedline may assert a length problem and affect the impedance the transmitter looks into when there is a reactive component
on the other end. Why? In this case, the load (on the feedline) is a
non-resonant length of wire, i.e., a reactive component. Therefore, the transmitter in
Figure 2 will only see 50 Ohms with a half wave, 50 Ohm
feedline connected. Why? A half wave feedline "repeats" at it's INPUT what it "sees" at it's OUTPUT and the >L< network for the "short" antenna
was designed to match 50 Ohm.
Conclusions . . .
Exclude... the feedline and it is noteworthy to say, the networks shown in figures 1 and 2 match the Smith Chart ' yin / yang ' curves,
i.e., there is most likely no point on the chart one or the other network configuration will not match to 50 Ohm.
Include... a feedline of random length and the >L< network may not tune all the bands, however, the operative word is "random" length feed
line. Change the feed line length a foot or two and try again.
Ah 'little Theory and Math (very little)...
The impedance presented by the end of a random wire can range over a very large playing field, from only a few Ohms to several thousand Ohms. This
calculation shows just one example, how to match a 50 Ohm source to a 377 Ohm load using the >L< network.
First find K
K = Square root of ((HighZ / LowZ) - 1)
K = ( ( 377 / 50 ) -1) ^ 0.5
K = 2.56
Next find X(L), reactance of the matching coil
X(L) = K times LowZ
X(L) = 2.56 X 50
X(L) = 128 Ohm
Next find the matching coil value
If X(L) = 2 Pi F L
then L = X(L) / 2 Pi F
L = 128 / (6.28 X 7.04E6) >>7.040 MHz<<
L = 2.89E-6 or (2.89 uH)
The series inductor should be approximately 3 micro Henry
Next find X(C), reactance of the matching capacitor
X(C) = HighZ / K
X(C) = 377 / 2.56
X(C) = 147.26 Ohm
Next find the matching capacitor value
if X(C) = 1 / (2 Pi F C)
then C = 1 / (2 Pi F X(C))
C = 1 / ( 6.28 X 7.04E6 X 147)
C = 1.5E -10
(1.5E -10) is 0.15 nano Farad or 0.00015 micro F or 150 pico F
For a lower SWR : > ) make C a variable capacitor and (if required) parallel fixed value capacitors to extend the range or series fixed
values to decrease the range (total value).
Thank You...
This article is the result of my response to a thread on qrp-l. Ade, W0RSP wrote in part . . . "If you really are worried about the weight of
a feedline, why use one at all? Instead put up an end fed antenna
coupled to a tuner". Doug, KI6DS, read my solution to the feedline weight problem and ask to publish my humble input. This is indeed an
honor, thank you Doug, for the invitation and privilege to publish in
QRPp. Special thanks go to Bill, KD7S and L.B., W4RNL, for their patient guidance and
knowledgeable assistance.
I welcome all comment, n3aaz-qrp@juno.com
John, N3AAZ
<<< END TEXT <<<
Figures
Notes:
Once you build this tuner, you'll need a SWR
indicator of some sort. For information on building a SWR Bridge to use
with this tuner, check out the following sites:
"Resistive SWR Bridge Notes" by N3AAZ
For these notes, please refer to the Resistive
SWR Bridge found on the G-QRP web page. This is an old tried and true way to measure SWR, it is simple AND IT WORKS.
The switch shown in that schematic can be eliminated. The BRIDGE is removed for QSO. Just install the BRIDGE between the radio and >L< network (or any tuner that does not have a metering circuit) then, adjust your tuner for minimum SWR then remove the bridge.
It is much more efficient this way since there is no 'RF sponge' is hanging on to your antenna.
Instead purchase a DPDT switch to install at the meter terminals as discussed below and
shown on the schematic.
This BRIDGE also forms a PAD (in this case, half power {3 dB} as discussed in my text to help protect the final in your radio when the SWR is high (just before the >L< Network {or ANY tuner} is adjusted for minimum SWR).
The diode shown is equal to the US 1N34 or it's equivalent.
The four resistors R1, R2, and R3 must be carbon and should be purchased all at the same time so they match. The exact resistance value is not critical, for example all four can be 47 Ohm and for this purpose (an SWR indicator) work just as well as 51 Ohm BUT they must be the SAME value and carbon composition for best results.
I build all my RF stuff over a copper clad board (apx 2 X 2 inch for this BRIDGE circuit).
Be sure to keep all leads on the board as short as possible.
The point marked >Z< on that schematic is DC and can be a BNC connector just like >TX< and >ANT< all (connectors) soldered directly to that copper clad board and then the pot >RV1< and the >METER< can be connected remote (via a short coax cable) to the BRIDGE which will keep all the RF stuff on the copper clad board. C2 is mounted on the board at the >Z< connector center pin. An additional capacitor (same as C2) can be
mounted directly across the meter terminals to help remove stray RF.
The meter polarity (this is a DC meter) on the schematic is not marked.
The meter terminal connections can be "reversed" with no problem and in fact used to your advantage, install the DPDT switch here.
One connection measures FORWARD voltage then a reverse connection measures the REFLECTED voltage
(VSWR).
PS... this bridge can be used to measure other things as well.
John, N3AAZ
<<< END TEXT <<<
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