Wideband antenna 50MHz-1GHz

[Solder_Junkie]

Like many radio amateurs, I would like to try measuring near field EMF levels. It is said to be difficult compared to using charts, or spreadsheets.

I have a calibrated hand held spectrum analyser, but lack an e-field antenna. Using a 6 inch antenna to obtain some idea of the levels involved in the near field of a 3.5 MHz dipole with 10 Watts, it produced easy to measure levels in dBm.However, how do you equate Volts per metre to the length of a short dipole?

[LaserSteve]

Try WA5VBJ's1 log periodic PCB antennas  for 300 and up.  You said broadband, you did not say FLAT...  Some of his receive well outside their range, like the big log periodic.

here is one of his efforts for ultra broadband.

https://www.wa5vjb.com/references/PlanarDiskAntennas.pdf

Steve

[Solder_Junkie]

These antennas are for 100 MHz and higher, I am looking to work out how to calibrate a small dipole for use on lower HF frequencies.

But thanks

[cdev]

Maybe a loop antenna would be best for you.

A HF dipole could be huge.

Lower HF antennas are large enough to make rotating them difficult at best. Would you want a helicopter parked in your back yard, with its rotating blades. A dipole might approximate that.

Calibration? I would expect them to vary significantly with  orientation and height.


For a stationary antenna, and a single, fixed direction, You might be able to extract the parameters with a VNA, at a fixed interval, say 1 MHz, and then use a spreadsheet to apply a correction factor "mask"-like over the raw DUT measurements to get the corrected response.

I suspect that would only work if the impedance was constant and known, which is unlikely, though.

 

These antennas are for 100 MHz and higher, I am looking to work out how to calibrate a small dipole for use on lower HF frequencies.

But thanks

The dipole likely will have a smoother response if you use the special balun which they use of biconical EMC antennas.

I don't know if you can approximate a biconical antenna with what you have, but I have attempted to build a great many broadband antennas in a repeatable manner, and I can assure you that the response of a vertically polarized bowtie or biconical or planar disk antenna (the last one, mentioned by Laser Steve above,  is a particularly easy and useful antenna, and very easy to construct, using two conducting circular disks, like pizza pans, and a stick. It eliminates the need for a balun if you properlyground he shield to the bottom, but it is finicky. You have to do it completely.

You could try a horizotal "bowtie on a stick" like that of T3sl4co1l Tim, above. - that bowtie shape I think could give you a more manageable antenna for any given frequency, size wise. Shortening its wingspan by at least 25%

With his or another balun, (i'm attachinga design below, this might work well, set to the 1:1 setting. It is likely to be much more broadband than any dipole you'll ever be able to make. Try it. Its a good start. Youre unlikely to find a way to fit more "antenna usefulness" into the space. isnt that usually the challenge?

Its not impossible to get an antenna that works over a 10:1 frequency ratio. 100-1000 MHz is not that difficult. And the construction is much easier than a discone. Make sure to use a balun,if you feed it with coax you'll have to.


Amplified active broadband e-field antenna.

As far as an e-field antenna, there you can utilize something like a mini-whip high impedance amplified antenna.

https://circuitsalad.com/2020/02/13/simple-low-current-mini-whip-antenna/

it will give you a good VLF antenna in a small space and may perform surprisingly well if you get it far enough away from your house and its noise field. It works on capacitance.

[LaserSteve]

Sorry, I missed a zero when I read your post.  Old Eyes do that.

NIST / NBS is your friend on calibration publications for antennas.  Here attached  is an example for a vertical monopole and a horizontal dipole. .  If you keep digging you'll find publications  for other designs / frequency ranges  as well as Reference antenna designs with known gain. 

Generally you can contact NIST's specialists with a little legwork and get your questions answered with a phone call or email if you are patient and persistant.  Other standards agencies will also have publications out.

They do have an antenna lab at NIST.

Keywords I used:  NBS antenna calibration   NIST antenna calibration.   NBS   STANDARD DIPOLE  NIST Standard Dipole and so forth...

You find things like this as well... https://nvlpubs.nist.gov/nistpubs/Legacy/TN/nbstechnicalnote1098.pdf

Most of this work would have been done back in the day when they were still "National Bureau of Standards".

Ultra-broadband amplifiers come from Amplifier Research, and mine covers 10 Khz to 1 Ghz at 50 Watts.    Their "Orange Book" is quite a good read.  If you do some digging over time  usually you can find an AR amplifier on Ebay for a fraction of the 3000$ going rate.



Steve

[cdev]

Have even seen multiple antennas used in the lab, the lowest band being a conical dipole or (planar) bowtie type.

I've even made this myself (apologies for the terrible photo),



just a space frame of wire in a flat bowtie shape, about 1m wide.  Works roughly 100MHz+.  But just for curiosity's sake, not for real testing.  (The balun if you were wondering, is fairly self evident: https://www.seventransistorlabs.com/Images/Bowtie_Balun.jpg )

Tim, your balun is electrically nearly identical to the Elecraft BN2 (in the 1:1 setting which is attached in PDF form below)

The real trick is calibration.  Test labs have the tools to do that.  Like, I have no clue what peaks and dips this thing has.  (if its similar to the Elecraft, its pretty damn good, basically its as good as it gets.)

If you need the confidence to do precompliance testing, you're probably better off buying a commercial one.  (Or go to a lab to calibrate your own construction, but that's probably as expensive, and won't look as nice.)

Tim

[Solder_Junkie]

Thanks Steve and Tim

Those NIST publications are really useful. I have a couple of home made active antennas, but my preference is for an electrically very short dipole of around 1/100th the size of a half wave one. On my hand held spectrum analyser the near field levels from 10 Watts fed into a half wave (full size) dipole on 3.5 MHz are around -50dBm to -20dBm.

The idea of the very small test dipole is (a) the levels are well within the range of the SA and are safe to measure, (b) the levels vary smoothly as I walk around the area of the transmitting antenna and (c) being electrically very short there shouldn’t be any unexpected “dips and troughs” in the response due to resonance, etc. The problem is one of calibration of the antenna factor...

[LaserSteve]

The NBS pub on UHF had something similar the excessively  short dipole of which you speak. I'm away from the work computer where I have Googles  AI
 trained to give me precisely what I want, well, as much as possible, but I'll try to find it.

Update, Try NBS technical note 1098.

The only reason I know about the NBS notes was a former employer wanted a field strength of X per
meter  over a buried wire for a test. Somebody asked him to have me play  as Team B for a sanity check.

 Otherwise this is not my field of interest, which is RF driven acousto-optics  Rf excited gas lasers,, and UHF/Microwave Ham.

Steve