Electronics > RF, Microwave, Ham Radio

DIY RF EMC Biconic Antenna

<< < (13/16) > >>

dazz1:
Hi
The method only works with two identical antenna, and it is not my method.  It is described in one of the MIL-STD461 versions.  Probably other references as well.
I can apply a known signal level from my sig-gen.  I normalize that via a direct cable connection to my SSA.  This effectively calibrates the SSA against the sig gen.
Setting the antenna a known measured distance apart referenced against calculated signal between two (theoretical) isotropic antenna. 
If the antenna are identical, then gain of each actual antenna will be the same.

nctnico:
But you still don't know what gets lost between generator - transmit antenna and receive antenna - receiver. For that you'd need to measure the SWR at your test frequencies and compensate. A while ago I read a report from R&S where they use 2 different antennas in 4 combinations in order to retrieve antenna gain versus frequency.

dazz1:

--- Quote from: nctnico on July 02, 2023, 10:47:12 am ---But you still don't know what gets lost between generator - transmit antenna and receive antenna - receiver. For that you'd need to measure the SWR at your test frequencies and compensate. A while ago I read a report from R&S where they use 2 different antennas in 4 combinations in order to retrieve antenna gain versus frequency.

--- End quote ---

I have a rf bridge so I can measure SWR.   If the sig-gen sends 10dBm to the antenna and 3dBm is reflected, then I know that 7dBm is being transmitted into the ether.  Both the Roberts balun and the antenna are entirely reactive (no power lost to resistance).   The same applies to the signal path from the antenna to the receiver (spectrum analyzer).

The unknown I am trying to find is the gain vs frequency of my antenna.   In an ideal world, I could mount an isotropic antenna (gain 0dB) on one end and my antenna on the other (at a known distance separation), and measure gain directly.  No one will sell me an isotropic antenna, not even R&S, so the next best alternative is to make the connection between Rx and Tx with a cable (gain = 0dB, no separation loss).    That provides a reproducible reference. 

So the problem is then that I don't have a calibrated antenna as a substitute for an isotropic antenna.   If I mount two identical antenna (known separation) with identical gain vs freq, then I can simply measure and calculate the gain.
If the matched antenna pair measure a gain of 10dB, then each antenna must have 5dB gain.  That is it.  I can then produce a measured, verifiable gain/freq curve for my antenna.

So I don't need a VNA to do any of that.  It would be useful to measure the impedance of the antenna so I can match the antenna to the balun. 

I do plan to use a 3rd antenna (any old coat hanger would do) specifically to look for any difference in the gains of my individual antenna.  If they are identical, the gain/freq curves will also be identical.   I am putting a lot of effort into ensuring the two antenna are identical.  I am working to far tighter tolerances than are necessary to build a biconic antenna.

So a VNA would be nice and I want one, I just don't need it for a one-off antenna project.

dazz1:
Hi
Another ants step forward with the antenna.
I glued in the spring detents with LocTite.

The photo show bits of element inserted to apply pressure to the spring ball detents to keep them in-place while the LocTite sets.  If they ever need replacing there are holes in the back of the bracket to push out the detents.

dazz1:
After waiting for the LockTite to set, I assembled the antenna.
It just snaps together so easy peasy to assemble/break down.  No threaded elements to turn into place.

The centre element is made slightly too long so it is clamped in place by the 6 elements pulling down on it.
All the parts will fit flat in a cheap Chinese gun case.

I still need to make the loop shorting bar that will also snap in-place.
I also need to figure out what balun to use, and then make it.  That is not a simple thing to do.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod