开云体育

This is what I do:

Connect the Vector Network Analyzer between your wire and a counterpoise and
sweep the frequency of interest, looking for X to be 0 (neither negative or
positive). If the value of X is negative, your observed frequency is below
the resonant frequency, if X is positive you are above the resonance. X at
0 (zero) indicates your resonant frequency.

You will probably need a counterpoise (ground plane, a wire parallel to your
antenna wire) for your single wire to work against. For purposes of
measuring element resonance I have found the ground plane should be
physically longer than your single wire. A little longer is fine and a lot
longer makes no difference in my experience. Just make sure the AA-600 is
seeing the wire and not the ground plane. Easy to tell, just touch the wire
and the frequency of X at zero ohms will (should) move.

After you get the zero ohms X value to your desired frequency then look at
the R value to know what you have to do to get your SWR to match your
transmitter.

Works for me. Paul, W8AEF

Pick a suitable range for the A-600, e.g. about 13~15 MHz for 20m and scan that range. You should see a dip in the graph pattern. Adjust the tuner for deepest dip (lowest SWR) which will be at the resonant frequency of the antenna. From that you'll be able to see if you need to lengthen or shorten the antenna. I would do this for all three bands just to see where you're at, then make the adjustments for the antenna segments starting at the highest frequency band. With the analyzer you'll also be able to see the interactions among the bands. Should be fun.
Hope this helps.
73,
Pat? ? VE3EUR

I borrowed an analyzer, a RigExpert AA-600.
hive-mind help needed.

how do you find the resonant length of a single wire, using a fancy gizmo? what I've tried so far is putting the wire in one side of a binding post-to-BNC connector, and no frequency anywhere near what I planned on had an SWR below the stratosphere. is there a different way to hook it up? so far, the demos I've found have people just screwing their feedlines' coax connector right on, no worries, but I'm trying to use this to find resonant lengths to assemble a multi-band EFHW antenna.

you might be mistaking my antenna for a different one. this little tuner is connected to my radio via an 8" pigtail.

All:
Conditions were not ideal, but we did get some RBN reports. While it's far too soon to say much, it appears the mag loop gives up about 1-2dB over the EFHW, which is up about 60' or so. VE8JY in Alberta heard us, but looking at their antenna system, they can probably still hear Jimmy Hoffa's last conversation. Also, we did get some rain during the test period which forced us to stop for a while. We tried to protect the cap, but were not totally successful. We are going to try again after we iron out some hiccups that showed up during the test. We also need to put it in a protective case.
About the design. The feed loop is rotatable (by hand right now), but we do not think it's worth the effort to motorize it. The software driving the cap via a stepper motor works pretty well, but we think we can refine it somewhat. We are about 50' away from the ML and the control aspects seem to work just fine. Why the 4-loop construction? Because we wanted to be able to fit the loop in the truck of a car without killing the efficiency. True, it's not an Alex loop, but we didn't intend it to be. On the other hand, it's going to be cheaper than most portable antennas and it can handle considerably more power than most. We have more tests in mind down the road, but we are encouraged by the results.
To all of you who tried to give us a listen today, Al and I very much appreciate your efforts on our behalf.
Jack, W8TEE

All:
Today we began testing Al's new mag loop (ML) design, shown here:

It's a 40-30-20M "luggable" loop. It's about 3' in diameter, breaks down so it fits in a car, and we can get almost 1.1:1.0 on all three bands with our remote controller. (Peter Parker might use it on the beach down under, but most of us won't use it as a walkable antenna.) We ran some tests today and did see RBN reports so we know it radiates. However, it's not waterproof yet and the rains moved in...again. We plan on testing later today starting at 3PM EST on 7.050MHz. We will be transmitting on two antennas: 1) the ML and 2) an EFHW in an attempt to get some meaningful comparisons. We expect to run about 20W of power. We will alternate between the two, but you won't know which is which. After about 45 minutes and it the weather holds, we will move up to 14.080MHz to repeat the tests.

If you have time around then, use the RBN to report what you're hearing. We'd really appreciate it!
Jack, W8TEEAl, AC8GY

Hi Lloyd,
Generally, you start adjusting the highest band and work down through the other bands but as, you discovered, with a multiband trap antenna there are interactions among the bands so tuning is tricky. Since it's easier to trim than to add wire, on the first pass try for resonance at lower than optimum frequencies and make note of how tuning a lower band affects the previously tuned higher band.
Although this antenna is a great idea, there is a flaw in the tuner design that might make it difficult to achieve low SWR on all bands. The tuner uses an autotransformer so the shield of the coax feed line is actually the counterpoise. That means the 10' counterpoise isn't a counterpoise at all because the feed line is. What you get isn't actually an EFHW antenna but an off center fed dipole. That makes tuning on three bands even more difficult.
The best way to tune an EFHW antenna, IMHO, is with a tank circuit coupled to a feedline with a secondary winding that isolates the feedline from the antenna. See <> for an example. For the QRP Guys antenna you probably need to tune the antenna with a very short feed line, say, 10'. Once you have all three bands in the ballpark, try a longer feed line to see how that affects the tuning. You might need to try different lengths of feed line to make the antenna work on all bands. An alternative is to modify the tuner to accommodate a true transformer rather than an autotransformer.
An antenna analyzer certainly makes the job easier because it tells you where the point of resonance is. Without it, you're confined to making SWR checks within the band. Perhaps you can borrow an analyzer from someone nearby.
Hope this helps.
73,
Pat Byers? ? VE3EUR

I'm wondering if someone can suggest a method, not involving using an antenna analyzer, to gradually construct this antenna so that it matches on all three bands?

I began by putting it together with the measurements suggested in the manual, and it didn't match at all on 20, and barely on 30.

Then I started again, tried a piece for just 20, got a match. Added a piece for thirty, and Trap A, and got a nice match on 30 - but no match on 20.

Help!

On Fri, May 24, 2019 at 04:49 AM, John KN5L wrote:

From this analysis, I see no benefit with the 12V version.

Yes but KK7B uses opamp biased to a 6V rail using 12V.
So yes at the CB end its not a big deal, at the other end of
the 3 stage all pass headroom is a good thing.

Allison

Because it's fun to do, and may as well complete project before cleaning
up, plotted dynamic range.



2N3904 gain plot added.

1dB gain compression is -21dBm input.

Measured down to about 1mV output. Below 1mV is both measurement and
noise limited using a simple breadboard construction. Demonstrating
about 70dB dynamic range, formalizing construction will increase
measurable dynamic range.

Measurement devices:
Rigol DG1032 with 40dB pad input dBm values.
Fluke 87V Hi-Res mode output voltage.

John KN5L

Hi Allison,

I was commenting about battery voltage range rather then current. Modern
three cell LiPos sag down into 11V range.

Some additional measurements at both 8 and 12 volt supplies.



For 12 V operation, change common base emitter resistor to 4.7k for 50
Ohm input. Ve=2.369V Ie=0.50mA. All else unchanged. Gain does not change.

Rough gain compression can be evaluated by adjusting oscilloscope input
and output traces on top of each other. Increasing input until output
trace sags with respect to input trace. Believe actual method is
measuring in an out voltages to find 1dB gain compression point.

Compression, using oscilloscope trace match described above is same for
both 8V and 12V versions, at about 12-13 mVrms input.

From this analysis, I see no benefit with the 12V version.

KK7B form is featured in ARRL "Experimental Methods in RF Design," page
8.13. Discussion for low-noise AF amplifier is limited to it having no
flaws and no additional design effort.

One flaw is CB emitter resistor for 50 Ohm input Z. Most everything
merits a revisit and possible improvement.

I hope Richard, original poster, is benefiting from these breadboard
measurements and study. Maybe reproducing these experiments for
conformation.

I found it to be quite educational! And fun to do.

John KN5L

Hi John,

I can see field friendly easily.
The MicroR2 does about 55ma for the whole receiver on 40M.
That is without CMOS opamps (uses 5532).

Allison

Found some new ON Semi 2N3904 ordered a few months ago.

New schematic in:

Using common part values.

Very close to Zin = 50 Ohm.

Moved output to mid Vcc, which increased gain some.

Visible oscilloscope clipping at about 17 mVrms input, 5.48 VPP output.

Assuming a 10-15 VDC field friendly rig with 8V regulated rail.

John KN5L

Look at KK7B R2/miniR2/MicroR2 as his solution for power was a capacitance multiplier
at 12V. Higher voltage allows greater dynamic range.

Looks like I wrote the wrong current down, though it should work there. At 12V a 1.8K
emitter resistor and the correct base bias to get .52ma + base current (.02ma or so)
will do fine. While common base is capable of very large voltage gain (basically ratio
of source impedance to load impedance) practical values are lower. Typical device
is 2n3904 or 4401 rather than a large device like 2n2222/2n2219 though they are quiet
the HFE is generally lower especially at low currents. the optimal device i on that offers
good gain (DC and AC) at under 1ma as well as being low noise.

Its a circuit I've built and used . I also use a variation as a bidirectional amplifer to
feed/and take audio from a DBM. In the feed the base is driven with audio and for RX
(take) the base in grounded (shut diode switch+DC block cap) and the audio is taken
from the collector with a 2.7K load.

I built and used all three multiple times. Image reject direct conversion has a lot going
for it in terms of sound quality. My 6M contest TRX is built around both teh RX and TX
(miniR2 and T2). Its features and reason for the design was 1KW+ 13db antenna 800ft
away usually aimed at me (that op runs over driven at that) so I needed a RX that
would not crunch! I am able to run within about 5khz of him if I ignore his buckshot
if he is on the opposing side band edge of me.

Fun stuff indeed.

Allison

Some additional comments:

Added KK7B form emitter follower, no change in input Z.

Input Z is a function of Vcc. Changing Vcc to 5V requires Emitter
current ~= 0.7 ma for 50 Ohm input Z. Emitter load R = 1.36k

A shipped application may benefit by using 5 or 8 volt three terminal
regulator.

Fun stuff!
John KN5L

Hi Allison,

Common Base Amp breadboard and measurements:


Emitter resistor adjusted for 50 Ohm input Z. Can't get much closer to
0.5ma.

KK7B form used 2.7k emitter resistor, Input Z was lower than 50 Ohm.


Has a section describing V gain is not predictable. I wonder if input
impedance is also component variable and requires tuning as done for
this experiment?

John KN5L

ajparent1/kb1gmx writes:

Paul,

For the case where the desired signal is audio and the DBM source is
a Common base amp is the desired case. For a mixer to RF then
other possible choice may have favor.

The OP was primarily addressing audio, such as a DBM being used for a direct conversion receiver (or maybe the product detector in a superhet). Though, I have little experience building direct conversion receivers, mostly superhets.

The KK7B form is Common Base with a emitter current of about .5 ma.
That will present a wide band load of about 50 ohms to the DBM and with
a 3300ohm collector resistor would net a voltage gain of about 66.

Jim Kortge, K8IQY, also used common base IF amps in his original famous 2N2/40 rig here:
On sheet 2, the Q5 CB amp matches the DBM to the XTAL filter impedance of 470 ohms, and the Q6 CB as an IF amplifier. Nice approach.

On my common base amps (for RF/IF), I also set Ie at .5mA to set Rin at 50 ohms and usually use a 1.2-1.5K collector load to limit gain to around 12dB for stability. Or, a toroid in the collector (like 10-12TP:3TS) if driving a 50 ohm DBM. I've also had good luck using the internal output transformer in a DBM as the emitter source for the CB amp by swapping the IF and RF ports on the DBM, such as the ADE series (the transformer is the RF port). Instead of grounding the bottom of that RF port transformer winding, that goes to ground through a resistor to set the emitter current. One of those ADE-1 ground pins is the bottom end of the RF port transformer.

the reverse isolation (S12) tends to be very good.

Isolation is certainly another advantage of incorporating an active stage following the mixer. It can tame an unruly leaking LO if that is a problem. On the ADE-1 (and most DBMs), the LO-IF isolation is 50-55dB and the LO-RF isolation around 60-65 dB, another advantage of using the RF port for the IF output.

Common emitter needs to run at much higher currents for a solid 50 ohms
in and the output must drive a 50 ohm load. The down side is with
resistive feedback you get noise. IF this is a DCRX or Image rejecting
DCRX that undesired added noise.

Or use a MiniCircuits 50 ohm MMIC if you have 50-100 mA to spare! :-)

A good discussion, Allison. A nice little tutorial and should give plenty of fodder to consider using either a common emitter or common base amplifier for matching to 50 ohms. There is no perfect solution, but lots of ways to skin a cat that works.

72, Paul NA5N

Paul,

For the case where the desired signal is audio and the DBM source is
a Common base amp is the desired case. For a mixer to RF then
other possible choice may have favor.

The KK7B form is Common Base with a emitter current of about .5 ma.
That will present a wide band load of about 50 ohms to the DBM and with
a 3300ohm collector resistor would net a voltage gain of about 66.
The noise figure of the amplifier is fairly small compared to feedback
amps and the reverse isolation (S12) tends to be very good.

Why .5ma... 26/ie-ma=Rin for an approximation.

Common emitter needs to run at much higher currents for a solid 50 ohms
in and the output must drive a 50 ohm load. The down side is with resistive
feedback you get noise. IF this is a DCRX or Image rejecting DCRX that
undesired added noise.

Allison

John,
Yes, that work if the output desired was the RF signal (SUM).
For audio the 100uh choke would have to be something like
10 Henry.

Allison

John KN5L writes:

A common 50 Ohm input impedance amplifier design evaluation:

I have used common emitter amps similar to the above with feedback and a stiff partially bypassed emitter with good success.

Another option I prefer is to use a common base (CB) amplifier. Characteristics are low-Z in, high-Z out with 10-15dB voltage gain depending on how it's biased. Thus, CB amplifiers work well as a low-to- high impedance converter with gain in both RF, IF and audio applications. I have used the CB amplifier on the output of a 50-ohm DBM mixer and 1.5K ohm output (collector resistor) for driving subsequent stages, and also to convert 50 ohm DBM output to 330 ohm crystal filter input. An emitter follower (EF) does the same in/out impedance conversion though with no gain. The EF and CB circuits seem to be highly overlooked in homebrew circuits where they can do the job well. At our HF frequencies, just about any old transistor would work. 2N2222s and a few resistors and caps are cheap.

72, Paul NA5N