# size voltage divider I need for transceiver to spectrum analyzer?

#### Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

•  Subject
• Author
• Posted on
I just picked up a small spectrum analyzer to check out some
transceivers I have.  They are 10 meter ham units and put out anywhere
from 4-12 watts @ 50 ohms.  The new spectrum analyzer can handle a
maximum of 10 dbm.  What size voltage divider would I need to reduce the
transceiver wattage to less than the 10 dbm for the analyzer?  Thanks.

Re: size voltage divider I need for transceiver to spectrum analyzer?
On 18/05/2021 7:10 am, Jason wrote:

You should be looking at the voltages of the tx outputs rather than the
power.

Re: size voltage divider I need for transceiver to spectrum analyzer?

1 watt is 30 dBm.  4 watts would be 36 dBm.  12 watts would be about
41 dBm.

In order to avoid driving the spectrum analyzer into overload, I'd
suggest keeping the input signal at about 1 milliwatt (0 dBm).  This
will also give you some safety margin to avoid exceeding the 10 dBm
limit.

40 dB of attenuation would take you down from 12 watts, to 1.2
milliwatts, so that's a good figure to shoot for.

So, you'd want either one 40 dB attenuator, or two 20 dB attenuators
in series.  Make sure that the attenuator is rated to handle the full
output of the transmitter, or you'll cook it.

A single 30 dB attenuator would keep you under your 10 dBm worst-case
limit if you feed it with 4 watts, but you'd be a hair over the limit
at 12 watts.  So, I'd stick with 40 dB of attenuation.

Make sure these are 50-ohm attenuators, of course, since that's what
the transmitters will expect (and want) to see.

Re: size voltage divider I need for transceiver to spectrum analyzer?
says...

To add to that, you probalby want to allow another 20 to 50 db so you
have plenty of head room so as not to over drive the SA and see spurious
signals.

A short antenna on the SA would probalby give enough signal to see what
is needed.

Re: size voltage divider I need for transceiver to spectrum analyzer?
Ralph Mowery wrote:
---------------------------------

**  Best a safest answer & "short" = about 100mm.

Either that or a single loop around the co-ax feeding a dummy load.

The SA needs to see about 100mV at 30MHz to stay clear of spuriae.

.....   Phil

Re: size voltage divider I need for transceiver to spectrum analyzer?
On 5/18/21 1:49 AM, Phil Allison wrote:

Tried the loop around the coax, just a prior loop I had made up one time
for RG-8 (this is RG-58 I'm using now).  It's not tight around the RG-58
of course, but output stays well within the analyzer limit.  However,
harmonics vary depending on where I move the loop.  Maybe I should make
a tight single loop around the coax?

Re: size voltage divider I need for transceiver to spectrum analyzer?
On 5/18/21 9:02 AM, Jason wrote:

at

e

RG-58

er,

make

Ok, as I already had a 20:1 voltage divider, which brought levels down
to 10 dbm, I simply added another one in series.  I've been testing out
a TinySA handheld unit about the size of a cigarette pack.  After
watching some videos on it, it is recommended to keep levels at -30 dbm
to prevent false spurs and harmonics.  My voltage divider added in
series does just that at just under -40 dbm.

Here's a question I can't seem to find the answer to: what are the
harmonic emission requirements for spectral purity for HF radio
transceivers in the US?  I *think* its either -30 or -40db below, but
it's been years since I've used these transceivers.  Thanks.

Re: size voltage divider I need for transceiver to spectrum analyzer?

Look in the FCC part 97 rules.  Such as here:

http://www.arrl.org/files/file/Regulatory/March%208,%202018.pdf

(d) For transmitters installed after January 1, 2003, the mean power of
any spurious emission from a
station transmitter or external RF power amplifier transmitting on a
frequency below 30 MHz must be at
least 43 dB below the mean power of the fundamental emission. For
transmitters installed on or before
January 1, 2003, the mean power of any spurious emission from a station
transmitter or external RF
power amplifier transmitting on a frequency below 30 MHz must not exceed
50 mW and must be at least
40 dB below the mean power of the fundamental emission. For a
transmitter of mean power less than 5
W installed on or before January 1, 2003, the attenuation must be at
least 30 dB. A transmitter built
before April 15, 1977, or first marketed before January 1, 1978, is
exempt from this requirement.
(e) The mean power of any spurious emission from a station transmitter
or external RF power
amplifier transmitting on a frequency between 30-225 MHz must be at
least 60 dB below the mean power
of the fundamental. For a transmitter having a mean power of 25 W or
less, the mean power of any
spurious emission supplied to the antenna transmission line must not

40 dB below the mean power of the fundamental emission, but need not be
reduced below the power of

Re: size voltage divider I need for transceiver to spectrum analyzer?
Jason wrote:
=============
Phil Allison wrote:

**  You using a proper 50 ohm dummy load or your antenna ?

Cos that result sounds like you have standing waves on the co-ax at harmonic frequencies.

......    Phil