Hwalker, ?JimAllyn & Phil Royce gave me VERY HELPFUL ANSWERS.

I have ordered the Nooelec SMA Attenuator Kit. ?Thank you for directing me there.

hwalker seems most clearly to understand my problem with attenuators. ?Yes, I may be over 70, but I'm still a student. ?I have a ham license, but I've yet to use it. ?What I have is a TinySA and I want to learn about is high frequency circuits using my TinySA. ?I also have a nanoVNA - F; as I understand it, the original concept came from the F?maker so I tried to reward that person with my money.

I seems clear, I do not want to stick an ohm meter to each side of my TinySA to find it's impedence; I strongly suspect NOTHING GOOD WOULD HAPPEN. ?Thus my poorly worded query about high power attenuators. ?My first thought if I were to encounter something high power is to make a voltage divider and tap off it at a lower voltage and thus not to over power my TinySA. ?So far I have put an antenna on it and observed my Baofeng UV-5RV+ and some local radio stations. ?I'd like to explore more but?? ?As I understand it, an attenuator is used to have the highest signal level on the screen without over loading the instrument. ?Why then to attenuators come with 'wattage'? ?Does an 100 watt attenuator absorb the 100 watts and leave a minuscule milliwatt signal at the output? ?Looking on the web, most appear to be nothing more than a pi filter. ?Further, you do not even need to do the calculations as there are sites that do the math for you.

I thank you all for your patience with the unwashed. ? ? ? ? ? larry

On Mon, Jan 11, 2021 at 09:13 AM, Lawrance A. Schneider wrote:

Does an 100 watt attenuator absorb the 100 watts and leave a minuscule milliwatt signal at the output?

In short: Yes.
The attenuator is showing its 50ohm resistance to its input. All power, except the small part that is transferred to its output, has to go somewhere so it?becomes heat.



?
--

Larry,
The wattage rating on the attenuator is the power handling capability of the device. Say you have a 30dB attenuator? and apply 100W, 100milliwatts comes out the other end and the rest is dissipated as heat in the attenuator. The 100 milliwatts is +20dBm and is still far too much power to apply to any test equipment. You would then cascade another attenuator of as much as 30dB to get to -10dBm which is a safe level to apply to most test equipment, but the power rating of the second attenuator could be minuscule, since it only sees 100milliwatts. Make sure to connect the high power attenuator to the transmitter under test first. Now another situation is say you have a 10dB attenuator and want to apply 100W to it, 90W is dissipated in the attenuator and 10W comes out the other end. You still need a 100W or larger rated attenuator.
Gary
W9TD

Try Barry Industries for attenuators, flanged, and possibly coaxial as well.

Out of Attleboro, MA.
You can also try Mini-Cricuits for coaxial attenuators. BNA, N, SMA Etc....

On Mon, Jan 11, 2021 at 10:44 AM, ERNEST AEC-RADIO wrote:

Try Barry Industries for attenuators, flanged, and possibly coaxial as well.

Out of Attleboro, MA.
You can also try Mini-Cricuits for coaxial attenuators. BNA, N, SMA Etc....

Thank you Gary,

I don't want to waste space so I'll not quote you entire missive.

So, it would be a good idea to buy a 100M 30dB attenuator and plan on adding some much smaller attenuators on the end with each having 50 ohm inputs and 50 ohm outputs at a more reasonable wattage. ?As long as the input/output are 50ohm, I can chain as many as I want. ?Were the signal were too small, reduce the chain appropriately? ?i.e. ?Start out with a long chain and reduce until the signal is large enough to give full screen height to the largest signal. ?I can worry about insertion losses after I have an idea as to how much attenuation I need?

Thank you very much both Erik and Gary, ? ? ? ? ? larry

I would recommend different approach - instead of buying attenuator that can dissipate all the TX power (let's say 100W) and be able to present 50 ohm to TX and 50 ohm to tinySA, my approach is to by a good 100W dummy load (it's always useful to have, they can be found for cheep quite often), and then have a "special" kind of attenuator that presents high impedance on the input side (several kohms) and 50 ohms to the tinySA. You can easily scale and buy 2kW dummy load and just add additional attenuation to your tap, and get covered for your full legal limit needs :)

Such device (some call it a "tap") then dissipates very small power (well under 1W), is quite cheaper, and easier to (even) build as DIY project

They come as "resistive divider (or resistive leader) - basically two resistors (non inductive), the "bottom" one being 50ohm, the top one easy to calculate, or as inductive - current transformer - but more difficult to "understand" but there are good instructions how to do it



There was a recent thread on this group addressing both?

As Miro suggests, I would agree.? A Tap makes more sense to me and can be built from parts that in total cost less than $10 + shipping.

Here is a link to the one that I built:


73
Evan
AC9TU

An alternative to resistor based attenuators could be an attenuating coupler. Large power absorbing resistors are not needed. All that is needed is a small ferrite ring and enameled copper wire.?

See this article and some of the references at the bottom.?

Hi Dennis!
inductive coupler still requires dummy load, and if you have one, then your resistive attenuator does not need "large power absorbing resistors".

In another thread in this group someone posted this link to the excellent article on theory and practice of inductive couplers ()

Article in your comment is quite misleading if not wrong - inductive coupling is done by means of current transformer. If you leave secondary side of current transformer "open" (no terminator with resistor), in theory potential at it's ends will be infinite value and in practice can reach thousands of volts!

Also, reflected impedance in primary will be quite high and will cause significant voltage drop and dissipated power in turn. Simply said, current transformer MUST be terminated on the secondary side at all times!

The "fix" is surprisingly cheap and simple - just have properly designed resistor :)

Also, number of turns and selection of toroid material is not a random effort - just check the link above, and if you desire to learn even more, check the link in that document that takes you here?

I still don't know what are comparative advantages/disadvantages of "resistive vs inductive" high impedance taps. Resistive are easier to make and have wider bandwidth (when done correctly), inductive gives you galvanic isolation, but not sure if there is anything else.

Evan/Miro
Do you have an opinion about a tap as you suggest vs. sampling the signal. (few turns of wire around a through RF connection, and depending ?on the resultant Magnetic coupling inducing signal into the sampling loop)

I agree that direct attentuation does not make sense when any kind of power is involved.
?
--
73 de k1jbd
bammi

开云体育

Dumb question here....

You guys are trying to check the output from your transmitter, right?
Don't you have enough RF in your shack that you could just us the tinySA antenna
to get enough signal to check it?? It is a form of inductive coupling, no?

If you are checking a qrp rig, maybe wrap a wire around your dummy load to get
more coupling.
73,
Gary
WB6OGD

On Thu, Jan 14, 2021 at 11:27 AM, bammi wrote:

Do you have an opinion about a tap as you suggest vs. sampling the signal

My opinion is that it depends on what you are measuring ?I am not sure of the linearity of the antenna when spanning a hundred MHz. A direct tap frequency response can be measured with a VNA to validate there is no loss or gain as a result of the connection. ?The separate antenna could pick up other stray signals in the area and mix with the ones you are trying to measure ?

With the above said, most all needs of a ham could be done without the direct connection. Assuming you are not down to measuring less than 2 or 3 DB difference. This could be critical for the 3rd harmonic of some of the uBITX bands that should be verified before and after amplification.?

my opinion, so cold be in error.?

73
Evan
AC9TU

Thanks for the response Miro. I came across Don Jackson's article after I posted my remarks. A similar well thought out design for a high power rf sampler can be found in the May 2011 issue of QST on pages 52 and 53 in the Technical Correspondence section by Tom Thompson, W0IVJ.

The two couplers shown on Frank Dorenberg's page, N4SSP, that I previously posted probably would not work well at higher frequencies
because they neglected to include a faraday shield to prevent capacitive coupling to the?secondary winding and they also neglected to put a shunt?resistor on the secondary. I'd already seen the g3yhn.info pages.

It all depends on what your trying to do and what your trying to measure. As has been already mentioned, if you're just trying to measure relative levels and not absolute values you could just?use the tinySA antenna.

<bammi asked> Do you have an opinion about a tap as you suggest vs. sampling the signal.

That question was recently asked in another thread, not many responses, but among ones that did respond I'd say that:

• resistive
  • easier/cheaper for DIY
  • if you use quality resistors (low inductance) and if you have good way to test transfer function (nanoVNA for example) to do some fine tuning, can get wider and flatter frequency response
• inductive
  • half way to building SWR meter :)
  • IMHO looks cooler :)
  • the key benefit - galvanic isolation (high voltage DC on the coax/antenna/TX side won't "traverse" the current transformer

On Thu, Jan 14, 2021 at 11:37 AM, wb6ogd wrote:

Don't you have enough RF in your shack that you could just us the tinySA antenna
to get enough signal to check it?

Garry, yes and now, depends what you are measuring.
If you just want to see signal around the carrier (for example you CW, or how wide is your SSB, or how deep is modulation on AM, or haw flat you FM), then you are right.

I need it to measure harmonic distortions - emissions on multiples of the base frequency, and getting signal from "the air around" brings all random attenuations on different frequencies (transmitting antenna, walls, tinySA antenna, ... Much (MUCH) more deterministic with "physical"? tap?

On Fri, Jan 15, 2021 at 11:40 AM, Dennis Roth N3AZB wrote:

It all depends on what your trying to do and what your trying to measure. As has been already mentioned, if you're just trying to measure relative levels and not absolute values you could just?use the tinySA antenna.

Only if you are measuring relative levels of signals in a narrow band. As soon you have signal spread over 3 harmonics, you mast have physical tap

I did some thinking here, plus some math and testing, then I found an excellent youtube from W0QE describing all that was asked here :) The only reason NOT to check his presentation would be the time - two videos (Part 1 and Part 2) are 30 minutes each, and are well worth time spent!




For some of you too busy to check them out, here is the digest version :) Even if we assume that my digest will tell you everything you need to know, still go and check Larry's posts!!!

Let's assume you want your tap to cover some extreme cases - from 1W to 1500W. You also want to get signal from the tap that you can take to your instrument (tinySA for example) that's 50ohm. This will drive the need attenuate your signal by 40dB (too much and your low power will be to small, too little and your legal limit will fry your instrument). Without too much explanations (see Larry's Part 1), you will end up with resistor divider 2500ohm/50ohm. Given that 1500W@50ohm->270Vrms,?your 2500ohm will need to be 30W. Too much :)
If you scale down your expectations to use tap for up to 100W, you will deal with 70Vrms, and resistor will be 2W, more manageable.
NOTE: all the numbers are for measuring where SWR is very low. Increase SWR, voltage goes up.

Larry is showing examples where you can use Cap divider (or inductive) so you eliminated power dissipation only related to resistors, but both have quite frequency dependent "behavior".

In Part 2 he explains inductive coupling (the current transformer), impact of stray capacitances, sizing, ... I especially liked the part with "how to minimize stray capacitance" and different design approaches to ""extend the shield"

He also touched on impact of SWR, and that applies to all cases (R/L/C/inductive).

Through both parts he is EXTENSIVELLY using SimSmith - if not familiar, don't be scared, he's explaining what's being done.

If you like his videos, check other of his stuff, i just finished his video on designing CMC (Common Mode Choke) and finally understood some of stuff that was always in front of me, but never clicked!

I just need to find more hours in a day and to check all that's on his youtube channel :)

So, to SUMARIZE:

• resistive taps are quite OK, but come with significant challenges related to needed power rating of a resistor used if you are constrained with 50ohm output?
• capacitive and inductive dividers are not practical
• inductive coupling, when done correctly, is the best design to cover wide range of power and frequency
• neither (divider/coupling) should be used to measure power if SWR is not very low

?

Very good descriptive replies here.? Thank you.
Here's some How To:
Hayward and Larkin, "Simple RF Power Measurement, QST June 2001 describes a resistive 40db tap.
On my website you will find a simple resistive tap at part B.I.a. in the Dongles notes.

The ARRL 2020 Handbook section 25.0.10 (earlier editions also have this section) describes inductive couplers.? You'll see an inductive 40db tap in the Dongles section part B.I.b. of my notes.

I have shown some N2PK VNA measurements on these taps.? My Icom 718 power setting numbers do not match power out so in PWR Experiment: Measure Transmitter Output Power I show how to use the taps to tabulate transmitter power output for various settings.
Incidentally, the notes show how to make a plastic RF tight shielded box from an AC switchbox.
Hope this helps someone.

How about attenuators such a these? They are both 50 ohm and give you a lot of attenuation options.
--
Scott Richardson AA6U