I don't think using your 8640B as a signal source to read on a wattmeter
is going to cut it. +20 dBm is only 100 mW. Unless you have a
wattmeter that has a 100 mW or so range, you aren't going to get much
needle deflection. The lowest range on my Telewave wattmeter is 5 Watts.

Scott
N0EDV

Mike Andrews wrote:

On Tue, 19 Dec 2006 09:58:41 -0500, Straydog wrote in m:



On Mon, 18 Dec 2006, Mike Andrews wrote:


Some friends and I have been looking into making antennas for 70 cm.
and higher frequencies, but we don't have an MFJ-269; my MFJ-259 tops
out at 175 MHz. I _do_, however, have an SWR meter that supposedly
works up to 500 MHz( and maybe higher), and an HP 8640B that is good
to about 1.2 GHz.

If the SWR meter is sensitive enough, could I get at least ballpark
SWR values using the 8640B as a source,


You did not say what an 8640B is. Signal generator? Transmitter? Power
out?


Lab-grade RF signal generator, official range from 500 KHz to 1.024
MHz. In practice it's good from 455 KHz up to about 1.2 GHz. Includes
FM, AM, and pulse modulation, with either internal modulation at variable
frequencies and amplitude (AM) or deviation (FM) or external modulation.
Has a very nice 6 1/2 digit counter good up to 1.2 GHz. Output range
is from -130 dBm (100 nanoVolts) to +20 dBm (3 Volts) into 50 Ohms.

It's a _very_ nice piece of gear indeed. You can see a picture of mine
(and other stuff on the workbench) at

http://mikea.ath.cx/testgear/index.html.


Main thing is to get the "forward" power to read something on the meter
(preferably full scale, but even if you get half or quarter scale, then
you are able to get a rough idea of usabiltiy of the meter). Then see what
it shows on the "reflected" scale. Figure ver roughly if the reflected
power is half the forward (regardless of the actual location on the meter
scale), then you have about 3:1 SWR. Turn the meter around to get an idea
if the diodes (and associated circuitry) are well matched for forward and
reflected.


I have used a number of el-cheapo CB SWR meters even up on two meters and
they work surprisingly well, even more sensitive than ham SWR meters for
VHF-UHF, and not that far off of what the ham SWR meters report, and so
are good for handie-talkie measurements. So, yes, you should get useful
ball park figures.


That's good to hear. I got to thinking about what an MFJ-259 or -269
does, and decided that I only needed a sensitive-enough SWR meter that
would give reasonably accurate readings from 100 MHz to 1.2 GHz, since
I already had the signal source. Of course, the HP 8640B weighs about
40 pounds, and is a good deal less portable than the MFJ products, but
if I can get this working, I'll be able to do things it won't.


running the signal through
the SWR meter to the antenna? Or am I way off in left field?


Looks like I'm well inside the ballpark and that I have some design
and construction to do. This should be fun.

Thanks!

On Tue, 19 Dec 2006 18:36:43 +0000, Scott wrote in :
I don't think using your 8640B as a signal source to read on a wattmeter
is going to cut it. +20 dBm is only 100 mW. Unless you have a
wattmeter that has a 100 mW or so range, you aren't going to get much
needle deflection. The lowest range on my Telewave wattmeter is 5 Watts.

Agreed. I'm looking at some RF log-amp chips from various places and
some RF sensor chips I saw in a construction article I can't locate
right now, so that I can get something useful with signal levels in
the Part-15 range.

I don't know what I'll wind up with, but it'll be interesting. I'll
share when it's soup.

--
Mike Andrews, W5EGO

Tired old sysadmin

In article ,
"Mike Andrews" wrote:

Some friends and I have been looking into making antennas for 70 cm.
and higher frequencies, but we don't have an MFJ-269; my MFJ-259 tops
out at 175 MHz. I _do_, however, have an SWR meter that supposedly
works up to 500 MHz( and maybe higher), and an HP 8640B that is good
to about 1.2 GHz.

If the SWR meter is sensitive enough, could I get at least ballpark
SWR values using the 8640B as a source, running the signal through
the SWR meter to the antenna? Or am I way off in left field?

Mike-

I think you are on the right track with the RF log-amps.

Another idea - I understand the MFJ-259 has a standard SWR configuration
with a built-in signal source that uses an ALC to keep forward power at
the equivalent of full scale. Why not replace the 259 signal source
with the HP 8640B?

Fred
K4DII

On Tue, 19 Dec 2006 15:04:58 -0500, Fred McKenzie wrote in :
In article ,
"Mike Andrews" wrote:

Some friends and I have been looking into making antennas for 70 cm.
and higher frequencies, but we don't have an MFJ-269; my MFJ-259 tops
out at 175 MHz. I _do_, however, have an SWR meter that supposedly
works up to 500 MHz( and maybe higher), and an HP 8640B that is good
to about 1.2 GHz.

If the SWR meter is sensitive enough, could I get at least ballpark
SWR values using the 8640B as a source, running the signal through
the SWR meter to the antenna? Or am I way off in left field?

Mike-

I think you are on the right track with the RF log-amps.

Another idea - I understand the MFJ-259 has a standard SWR configuration
with a built-in signal source that uses an ALC to keep forward power at
the equivalent of full scale. Why not replace the 259 signal source
with the HP 8640B?

I hadn't thought of that. I suppose I could put a jack on the 259's
case, and a switch to select the 8640B, which has _marvelous_ ALC all
its own. The switch, and some RG-174, ought to be good enough for what
I'll be asking them to do. But that changes the question to "Is the
259's SWR sensor good enough above 175 MHz, and especially around 33
cm to 25 cm, to give real-world answers?"

I'll never know if I don't try.

I may have to buy a factory UHF-and-above antenna analyzer to validate
my answers, of course. I'm sure W5DNA will understand why we're eating
potato soup for two or three months.

Thanks!

--
Mike Andrews, W5EGO

Tired old sysadmin

I'd start by measuring a standard or two. You can get pretty decent
termination resistors on eBay for a modest cost. If your system measures
close to 1:1 when terminated with one of those, you're off to a good
start. Next I'd make up a 50 ohm microstrip line terminated with a 50
ohm chip resistor. If that reads close to what the 50 ohm termination
did, then I'd measure the SWR with some different load values, say 25
and 100 ohms. These of course could be made from two 50 ohm chip
resistors. You could get more elaborate with the calibration by
introducing reactive loads, but just the three data points should be
enough to give you some confidence in your SWR measurements, at least
near 1:1 where your primary interest is.

Roy Lewallen, W7EL

Just one comment, about halfway down....

On Tue, 19 Dec 2006, Mike Andrews wrote:

On Tue, 19 Dec 2006 09:58:41 -0500, Straydog wrote in m:


On Mon, 18 Dec 2006, Mike Andrews wrote:

Some friends and I have been looking into making antennas for 70 cm.
and higher frequencies, but we don't have an MFJ-269; my MFJ-259 tops
out at 175 MHz. I _do_, however, have an SWR meter that supposedly
works up to 500 MHz( and maybe higher), and an HP 8640B that is good
to about 1.2 GHz.

If the SWR meter is sensitive enough, could I get at least ballpark
SWR values using the 8640B as a source,

You did not say what an 8640B is. Signal generator? Transmitter? Power
out?

Lab-grade RF signal generator, official range from 500 KHz to 1.024
MHz. In practice it's good from 455 KHz up to about 1.2 GHz. Includes
FM, AM, and pulse modulation, with either internal modulation at variable
frequencies and amplitude (AM) or deviation (FM) or external modulation.
Has a very nice 6 1/2 digit counter good up to 1.2 GHz. Output range
is from -130 dBm (100 nanoVolts) to +20 dBm (3 Volts) into 50 Ohms.

3 volts into 50 ohms isn't much power. ~3/50 of an amp, means about 180
milliwatts. The most sensitive calibrated SWR meters will read zero to 5
watts full scale, so 180 milliwatts might move your needle for forward power
about 5-10 degrees of arc away from the leftmost peg towards the rightmost
peg on the dial. That doesn't give you much indication of reflected power
since a 3:1 ratio would then mean movement of the needle about 2.5 to 5
degrees of arc, respectively. The elcheapo CB SWR meters _might_ be more
sensitive since more power _should_ transfer to the measuring bars because
2 meters is some 5 times higher in frequency and the CB SWR meters are
meant to operate at max 3-4 watts of RF output from the CB rigs. But,
you'll have to try it.


===== no change to below, included for reference and context =====

It's a _very_ nice piece of gear indeed. You can see a picture of mine
(and other stuff on the workbench) at

http://mikea.ath.cx/testgear/index.html.

Main thing is to get the "forward" power to read something on the meter
(preferably full scale, but even if you get half or quarter scale, then
you are able to get a rough idea of usabiltiy of the meter). Then see what
it shows on the "reflected" scale. Figure ver roughly if the reflected
power is half the forward (regardless of the actual location on the meter
scale), then you have about 3:1 SWR. Turn the meter around to get an idea
if the diodes (and associated circuitry) are well matched for forward and
reflected.

I have used a number of el-cheapo CB SWR meters even up on two meters and
they work surprisingly well, even more sensitive than ham SWR meters for
VHF-UHF, and not that far off of what the ham SWR meters report, and so
are good for handie-talkie measurements. So, yes, you should get useful
ball park figures.

That's good to hear. I got to thinking about what an MFJ-259 or -269
does, and decided that I only needed a sensitive-enough SWR meter that
would give reasonably accurate readings from 100 MHz to 1.2 GHz, since
I already had the signal source. Of course, the HP 8640B weighs about
40 pounds, and is a good deal less portable than the MFJ products, but
if I can get this working, I'll be able to do things it won't.

running the signal through
the SWR meter to the antenna? Or am I way off in left field?

Looks like I'm well inside the ballpark and that I have some design
and construction to do. This should be fun.

Thanks!

--
Mike Andrews, W5EGO

Tired old sysadmin