log periodic antenna reference point

Hi members

I am working on log periodic antenna design. someone plz help how to determine reference point for distance measurement on antenna for testings.
I have seen reference point mentioned on antenna manufactured by ETS near the end of log-periodic antenna.

Thanks

you should avoid near-field region

while some antennas, like a horn, dipole, or monopole will have a fixed phase center, broadband multielement antennas like a log periodic do not. The phase center changes as a function of what frequency you are at.

I think, effect can be easily visualized for a LP antenna, because it's fed by a travelling wave spreading against the radiation direction.

Sometimes on antenna drawings a reference point, or "datum" is specified. Is that what you are talking about? That's up to the individual. I'm not sure what context you are talking about. For a log-periodic antenna I personally might consider the reflector as a reference point, and measure all distances from there. But there's no reason one could not defined the last element, or the start of the boom, the end of the boom... or whatever else one wants to define.

You need to clarify the context of your question for me.

Thanks all for ur replies
With reference point I mean in some testings standards suggest some particular distance of DUT from the antenna like in biconnical from edges and loop antenna from center, so my question was from which element to measure distance in lopg-periodic case and as I mentioned I have seen antennas labelled the measurement point in log periodic(It is not at last element). And as u r describing it might be a reflector. But I am unaware of the reflector concept in log-periodic. Can u describe briefly what is reflector and what makes u select it as reference point.

In my view, there's no reflector. For the determination of an effective reference point, I would refer to the below explanation in Balanis Antenna Theory, 3rd edition:

Thanks all for help but I am still unable to comprehend or determine a logic to mark a fix point (as refrence point) for all frequency band of a log periodic antenna, which Experts plz give some detail explanation.

All contributions as well as the quoted literature suggest that there can't be a fixed frequency independent reference point for LPD anntennas.

Do you have any references with different claims?

Most likely, the ETS "reference point" is valid for *their* cited or measured gain purposes when their antennas are used in radiated EMI/RFI testing.

And then, do not confuse their method of 'reference point' with the methods used in describing or designing Log antennas; by using the 'reference point' that is in the 'active' region of the antenna (see Balanis Antenna Theory, 3rd edition definition posted above by FvM) one will be 'true' to the governing physics, as opposed to just specifying a convenient physical point while in the anechoic chamber as ETS specifies (the technicians in the chamber need a simple 'marker' point to work with; they are not usually antenna engineers!).

Jim