[Elecraft] Antenna question

[Ralf Wilhelm]

Hi Ed,

Actually, my S9+67 dB for 0dBm turned out to be wrong - I lost one "6 dB" in my book keeping, so 0dBm is S9+73 dB - or S9 equals -73 dBm (which is the number I had in mind).

No doubt about the space loss formula and the far field  - "free space loss" is no dissipative loss like for example loss in the ground but just a "geometry factor". Basically the free space formula calculates the "aperture area" of your rx antenna divided by the area of a sphere centered on the tx antenna and with radius r (r being the distance). But this is the "free space loss" only if E and H fields decay as 1/r because that means that power density decays as 1/r^2 and integration over the sphere yields the total emitted power (which is the idea behind the formula) and by definition 1/r decay is the far field. If the 1/r^2 and 1/r^3 components of the fields have not already decayed, the space loss formula gives wrong results (which is why the german "FCC equivalent" wants us to either perform a near field calculation or  a measurement if we run more than 10 Watts ERP to be sure that we don't exceed the near field limits).

Regarding the "polarization decoupling" you mentioned: one can see by a little "gedankenexperiment" (as we say in physics even in english) that all electrical field vector components exist in the near field of the transmitting antenna: a part of the near field of a (electrically short) dipole follows the charge distribution on the dipole immediately (almost no "retardation") and looks pretty much the same as the field between a positive and a negative charge (a static dipole) - or a short magnet - there are some field lines leaving the positve charge perpendicular to the antenna axis and then bending towards the other charge. If you view this from the side, you can see that even a perfectly horizontally polarized antenna has a vertical near field component, but for a short dipole, this decays as 1/r^3 (I think so) and is not present in the far field any more but might be dominant in the near field - so, with the vertical in the near field, the polarization decoupling can be drastically reduced. Only if the "yagi plus vertical system" is symmetric - the (balanced) yagi pointing exactly towards the vertical or exactly  in the opposite direction - this near field component will vanish (for symmetry reasons) and the polarization decoupling works, but for the other directions of the yagi it won't...

So, regardless if you simulate or measure, don't forget to turn the yagi antenna into several directions - may be the coupling is stronger when one end of the driven element is close to the vertical...


Greetings

Ralf, DL6OAP





Am 12.02.2014 um 21:46 schrieb Edward R Cole <kl7uw at acsalaska.net>:

> Ralf,
> 
> Thanks for catching my mistake in the ohm's law formula.
> P = E^2/R
> 
> Regarding using the far-field loss formula it is probably prudent to try measuring power on the receiving antenna when transmitting on the other antenna (the yagi).  At the very least check with a SWR meter in the lowest power range and see if you detect anything.  If the meter deflects or "twitches" power is probably way too high and you need some kind of protection device.  There are some simple milliwatt power meter designs in some ham Handbooks (look for field strength meters); simplest is a IN34 and 1ma meter.  If you blow up the 1N34 you have your answer!  Better that you use a couple 20-dB coax attenuators before the meter at first.  I no power is seen then remove one and test again.
> 
> You can rely on using 0 dBm as maximum survivable input to the receiver, but the receiver still will be driven into compression and not usable while transmitting.
> 
> My example of 130-feet was at 144-MHz so not a fair comparison with HF freq. which have much longer wavelength.
> 
> The space loss formula is useful for making measurements at far-field (google it)>
> 
> 73, Ed - KL7UW
> 
>