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eHam Forums => Elmers => Topic started by: KILLINGTIME on March 07, 2010, 11:10:53 AM



Title: Using a field strength meter..
Post by: KILLINGTIME on March 07, 2010, 11:10:53 AM
Hello,

I've got a few portable radios (hand held), and was wondering how one might measure the output power.

[Please feel free to correct my understanding below if necessary]

After reading some posts on here, it would appear that measuring the power delivered by the RF amplifier of a hand held is not a good indication of how well the device will perform in the field, because of the compromised antenna system in use (very small, capacitively coupled to ground, objects in close proximity to the antenna etc).

It would appear that, the power of a hand held RF amplifier, and it's radiated power are two different things (antenna efficiency not being 100%). So, it makes more sense to measure the radiated power - with the use of a signal strength meter.

I've had a look on the net, and there are a few DIY designs around, and you can buy one for about 50 USD.
In the most basic form, they appear to be capacitively coupled telescopic antennas, which rectify and then smooth the received RF (no selective tuning). The DC is then used to drive a sensitive coil meter (uA).

I understand that the above cheapo FSM are only good for measuring relative output (ie no calibration).

My question is: how do you get meaningful relative readings out of such FSM devices?

http://www.mfjenterprises.com/Product.php?productid=MFJ-801

I've downloaded the instruction manual that's provided with the above meter, but I think there's more to it than just holding the meter a few meters away from your antenna.

Lets say I wanted to measure the radiated power of various hand helds (relative to each other), at different times of the year, as they come into my possession. These might be radios on loan from other HAM enthusiasts. The point here is, I might not have access to all radios at the same time. Access would be to different radios at different times of the year.

The above FSM relies on capacitive coupling for ground, and so do hand held radios.

[Correct me if I'm wrong here]

In order to get meaningful relative readings you'd have to:

1. Use the same location for measurements each time (an empty farmers field say), as even selecting the same distance between radio and FSM at a different location would invalidate any previous reading due to nearby obstacles (trees etc).

2. Ensure the same people holding the radio and FSM on the reference test, are used on all subsequent readings. Given each device uses capacitive coupling to ground, how they hold each device, and the type of clothing / footwear they use will affect the impedance to ground, and therefore the reading on the meter. [I'm not sure if different people - weight - size - perform differently as a path to the ground plane].

3. Ensure the same frequency is used. I don't see any doco for frequency response curves, so unless the response is flat across the whole spectrum, testing a UHF handy against a VHF handy at a later date is going to be flawed.

4. Ensure the same weather conditions are present. Ground that's soaked with rain water I assume will have differing ground plane properties to very dry ground. This will affect the reading again.

For 50 USD, I don't expect miracles, I'm just curious as to what I am getting. If it's just a meter needle that moves a bit every time I key up, then it's not much use.

I suppose the only way the above meter would be of any meaningful relative use, would be to calibrate the FSM each time before use, with a known standard - like another hand held that uses the same frequency as the device under test. That way you'd be using the same person / location / frequency.

Is this correct?

Thanks.


Title: RE: Using a field strength meter..
Post by: WB6BYU on March 07, 2010, 11:59:11 AM
I tried measuring 2m antennas with a Field Strength meter just last week.  I had
a number of mobile antenna whips, many unmarked, that I wanted to test.  With
the HT on low power the FSM had to be within 1 foot of the antenna to get an
indication of 20% of full scale.  Since some of the antennas were taller than that,
the results don't necessarily correlate well with the results at further distances.

If you run the HT at full power, and attach a beam antenna to the FSM instead
of the standard whip, you probably could get a usable measuring system.
(You may have to add a transistor amplifier to the meter for more sensitivity
if it doesn't already have one.)  Create a standard test fixture for the HT - for
example, sitting on top of a specific fence post.  Set up the FSM about 10' away
with the beam pointing at the test site.  Ideally you would make it so that the
meter reads nearly full scale with the strongest signal you expect to measure.
This will give you a relative indication that one radio radiates a stronger signal
than another.  For more precise measurements you can use a step attenuator
between the beam antenna and the FSM to mark the dial face in 1dB steps.

There are still a lot of potential accuracy problems with this system: the FSM
response curve changes with the sensitivity setting due to the forward voltage
drop of the diode.  Also you have to put the HT on transmit, then walk to the
other end to read the FSM, and the signal strength will change as you walk
around the equipment.  But it should give you a reasonably repeatable method
to check that a particular combination of transmitter and antenna is putting
out about as much power as other similar combinations do.

My usual method for field strength measurements is to use a standard receiver
with a step attenuator.  I set the step attenuator to a specific mark on the meter
(often whatever it defines as "S9") then do the same for the radio and/or antenna
being tested.  The difference in the attenuator settings for the same received
signal strength is the actual dB difference in radiated signal level.


Title: RE: Using a field strength meter..
Post by: N3OX on March 07, 2010, 12:15:17 PM
The step attenuator + receiver technique is probably one of the more accurate ones.

If you really want a stand-alone meter, I'd look at projects that use something like the Analog Devices AD8307 log amp:

This one is a "power meter" but when coupled to an antenna instead of a transmission line coupler it could do field strength duty:

http://n2pk.com/RLPmtr/RLPv1c.pdf

Here's one that is designed as a field strength meter:

http://www.utaharc.org/rptr/wdr_fsm2.html

To limit the effects of the measuring device's antenna, I would probably recommend using a good vertical  dipole with a non-conducting support tuned to the frequency of interest as your test antenna, and I would use a display that can be read from a decent distance.    If you choose to go a route that has an antenna feedline, make sure it is choked very well and routed appropriately.  A battery powered small meter would be better in this sense because the physical layout is easily reproducible.

I think you're right that you should use the same person (and probably take several readings with your test subject facing different directions)

If you wanted to get really fancy and have a controlled experiment with a "body" in place, you could rig up a mounting point on a plastic trash can on a rotating platform or something that you could fill with slightly saline water of known salt concentration.

If you want to mitigate the effects of the ground, finding a couple of elevated locations separated by a suitable distance could help.  Check this out:  http://en.wikipedia.org/wiki/Fresnel_zone

Otherwise, you do need to control ground reflections a certain amount.

One way to control for these problems is to also use a half wave dipole with a known signal source as a transmitter reference signal... then you always know that 500mW to your rubber duck is so many dB down from 500mW to a half wave dipole in the same conditions.  That should fix a lot of the environmental variation problems.

Eliminating the effects of the often single-ended design of field strength meters and the resultant dependence on coupling to whatever can go a long way to helping th reliablity of the measurements.

73
Dan








Title: RE: Using a field strength meter..
Post by: K8KAS on March 07, 2010, 12:19:35 PM
A Field Strength meter is a BASIC in most Ham shacks. It for the most part it is a relative reading used to measure RF field, I measure FS when tuning up my mobile
antenna (tune for max) and to look at patterns off the side of the car when keyed with a steady carrier/string about 15 feet long to keep a constant distance from the antenna. I won't go into calibrated
lab types used on antenna ranges, thats not what you get for $50. Again it is a simple RF field testing device no more no less.


Title: RE: Using a field strength meter..
Post by: N3OX on March 07, 2010, 12:31:05 PM
I won't go into calibrated
lab types used on antenna ranges, thats not what you get for $50. Again it is a simple RF field testing device no more no less.

But you *can* get much better accuracy at home .. the step attenuator method is excellent and basically costs the price of a step attenuator if you already have a suitable radio.  That could be up to $80 or $90 if you buy new, but if you're lucky you can get one much cheaper (mine was free, a pull from an old broken piece of test equipment)

For measuring antenna patterns and even relative gain, I use my radio and my laptop's sound card with G4HFQ's Polar Plot software, which is free.  VK1OD has some software called FSM that is for making absolute field strength measurements:

http://vk1od.net/software/fsm/index.htm

I didn't concentrate on these techniques because they require a SSB receiver. 

A little simple analog FSM is a handy device to have around, but I don't think I'd use it for this purpose.... however, you don't have to go with some thousands-of-dollars lab equipment to take the next step.

73
Dan


Title: RE: Using a field strength meter..
Post by: AA4PB on March 07, 2010, 01:25:37 PM
The Field Strength meter at a set distance is fine if all you want to know is which radio has the higher Effective Radiated Power (ERP). If you want to know how much more power then you need better equipment.

One of the best ways is to have a receiver with a signal strength indicator (S-Meter) on it. Put a good attenuator between it and the test antenna. Now you have a sensitive device so you can move much farther away, getting beyond the near field of the antenna. Key transmitter #1 and adjust the attenuator for some reference signal level like S-9. Now key transmitter #2 and adjust the attenuator the the same reference signal level. The difference between the two attenuator settings tells you the difference in the received signal in dB.

You need a pretty clear "test range" (open field) otherwise you'll be dealing with reflections off buildings and people that will through off your measurements.


Title: RE: Using a field strength meter..
Post by: WB2WIK on March 07, 2010, 06:00:27 PM
The problem with typical FS measurements is they're not necessarily indicative of what works best.

AM BC stations take such measurements at near ground level, since they're expected to generate a ground wave signal.  We're not.

In many cases, what really counts is how strong your signal is at 20-30-40 degrees above ground, and unless you have a helicopter, these measurements are difficult to make.

I wouldn't rely on a FS meter to tell me much of anything, even if it was very accurately calibrated.

As an example, a FS meter here in my shack might indicate a relative reading of "5" on a 10 scale when everything is working perfectly.  

Then, one end of my dipole falls down, and is now laying on the ground.  The stations I can work on the air is a greatly diminished quantity, with much reduced signal reports.  However, my shack FS meter will actually indicate a higher reading, because one end of my antenna is now much closer to the meter.

This isn't theoretical, it really happens and it's easy to demonstrate.


Title: RE: Using a field strength meter..
Post by: N1LO on March 07, 2010, 08:51:00 PM
I use a FSM regularly on 2m to demonstrate the effectiveness of adding counterpoise wires to the antennas.

The first thing you want to do is retrofit the FSM with a female BNC chassis jack. Then you can plug in antennas or coaxes.

To make the FSM more sensitive on 2m, I attach a 2m HT antenna with counterpoise wire right on the bnc jack.

If you build a little 2m dipole on a stand, you can run the coax through a step attenuator, then to the FSM and start cooking with gas! Oh, sorry, I mean then you can make really meaningful comparisons in dB.

Punch in attenuation until the stronger antenna gives the same reading as the weaker one and read the difference in dB.

--...MARK_N1LO...--


Title: RE: Using a field strength meter..
Post by: G8JNJ on March 08, 2010, 06:01:48 AM
Hi,

If you use Mark's method (which is perfectly valid)make sure you have at least 6dB of attenuation switched in at all times. By doing this you ensure that the FSM and the antenna it is connected to, are always terminated with something like a 50ohm resistive load.

Otherwise your readings will vary as the load impedance changes when you switch in less attenuation.

Regards,

Martin - G8JNJ

www.g8jnj.webs.com


Title: RE: Using a field strength meter..
Post by: KILLINGTIME on March 08, 2010, 10:55:28 AM
Thanks for the replies.

I knew there was more to using these devices than was published in the manual.


Title: RE: Using a field strength meter..
Post by: WB6BYU on March 08, 2010, 11:25:32 AM
For adjusting a mobile whip for maximum output - where the positions of the meter and
the antenna are constant, and the meter is within the near field of the antenna - they
work OK.  But testing a VHF HT adds a number of quirks.

Actually, one aspect that has been mentioned only in passing is the ability to read
the meter from some distance away - especially if you have to be squeezing the PTT
switch on the HT while taking the measurement.  One solution is to build your own
using a large meter face - I've got one about 6" across that came out of an automotive
tester that should be readable from 10' or more.  Another option is an LED bar graph
display, or using the meter output to vary the pitch of an audio oscillator driving a
speaker.  I find this later approach very convenient:  I use it for indicating the signal
strength when transmitter hunting, but the ability to know what makes a signal
stronger or weaker while keeping your eyes on the adjustment you are making (or
where you are running) makes life a lot simpler.

You may need to build your own meter to incorporate some of these approaches -
it isn't difficult, and you can customize it to meet your needs.  I'd probably use a
coax input jack, tuned circuit, rectifier, and a DC amplifier to drive the meter with
more sensitivity.  A CD4046 IC chip makes a very simple audio oscillator, driving a
LM386 or LM380 audio power amplifier to a speaker.  Probably want some type of
gain control for when signals are strong, too.  It would be a great test instrument,
but not something that there is likely to be enough demand for to find off-the-shelf.


Title: RE: Using a field strength meter..
Post by: KL7AJ on March 08, 2010, 11:41:59 AM
Measuring absolute field strength accurately is very difficult.  Fortunately, measuring RELATIVE field strength is extremely easy!

About 99% of measurements you will make in amateur radio will be RELATIVE measurements....is what I'm doing to widget A making Widget A better or making Widget A worse?  The cheapest receiver imaginable will tell you this.  This goes for almost any other measurement you can make as a ham.

About the only time you need an ABSOLUTE measurement is when you start pushing the legal limit of an amplifier, and need to know that you aren't exceeding the FCC limint, or crowding the band edge, where you need to know your absolute frequency! (You shouldn't do this anyway)

Hope this helps.

Eric


Title: RE: Using a field strength meter..
Post by: VK1OD on March 08, 2010, 11:46:54 AM
...
Actually, one aspect that has been mentioned only in passing is the ability to read
the meter from some distance away ...

One solution I use is to terminate a known antenna with a known detector driving a battery powered LCD panel meter. The detector / display can be made small (in HF terms), it has no feedlines or power cables to create common mode issues, and can be read with a telescope or binoculars at some distance.

Calibrating the detector (DC out vs PowerIn) is easy if you have access to an SSG.

I have used this technique successfully for a survey of field strength on 20m and 40m using Small un-tuned single turn square loop for field strength measurements (http://www.vk1od.net/antenna/SmallUntunedSquareLoop/index.htm) to make absolute field strength measurements. The equipment is light enough to raise it on a non-metallic pole for above ground measurements. My interest at the time was validation of NEC models of antenna behaviour.

Some may question the accuracy of the absolute field strength measurements made in this way. The loop's performance has been compared to a commercial calibrated loop in A/B field measurements and was very close. Field strength measurements are always a problem, and standard error of around 6dB is common with commercial equipment.

This solution could be applied to a half wave dipole on 2m (as suggested by Mark), but there are a host of issue with an open area test site on 2m as noted by others.

As noted, field strength is a three dimensional property of an antenna system, and optimising it at ground level (because of accessibility) isn't necessarily the end objective.

Owen


Title: RE: Using a field strength meter..
Post by: KL7AJ on March 08, 2010, 01:50:07 PM
OD;
"..I have used this technique successfully for a survey of field strength on 20m and 40m using Small un-tuned single turn square loop for field strength measurements (http://www.vk1od.net/antenna/SmallUntunedSquareLoop/index.htm) to make absolute field strength measurements. The equipment is light enough to raise it on a non-metallic pole for above ground measurements. My interest at the time was validation of NEC models of antenna behaviour..."

This is precisely the method we used for verifying the field intensity of our Electromagnetic Pulse (EMP)Weapon...er...DEVICE we were developing at Hipas Observatory. (We weren't officially allowed to call it a weapon, but that's what it was.  Very effective for what it did, too!  We could peel the traces off a microwave stripline circuit board at 1000 paces!)

Eric