[Elecraft] Antenna question

[Fred Jensen]

On 2/11/2014 3:36 PM, George Thornton wrote:
> This might be a stupid question, but here goes.

Only unasked questions are stupid.
>
> I have a small lot.  I currently am using a 3 element Yagi that
> barely fits on the property.  I was thinking about getting a vertical
> as a second HF antenna.  If I put it up it is going to have to be
> pretty close to the Yagi.

You will likely find that a vertical <anything> will be 1-2 S-units 
noisier than your horizontal yagi [or any other horizontal antenna, in 
general], *unless* you have essentially *no* man-made noise.  You said 
"small lot" which suggests an urban or suburban environment.  I have a 
tri-bander, a large sloping Vee for 160-30, and a GAP Titan vertical on 
the pipe that carries my coax up to the roof and to the other antennas. 
  I can almost always count on the GAP being a couple of S-units noiser 
than the other two on any of the bands they work on.
>
> I have a dual receive K3.  If I have both receivers going, one on
> each channel, and I broadcast on one of these antennas, am I in any
> danger of overloading and frying the other receiver?

Antennas <all> have a near-field and a far-field and the definition of 
"near" and "far" depends on the operating frequency, among some other 
things.  The lower the frequency [longer wavelength], the farther "far" 
is from the antenna.  If a second antenna is located within the 
near-field of the first, they will become a coupled system ... the 
second antenna will behave as if it was an element or elements of the 
first antenna.  In this case, some fairly large amounts of RF power can 
show up at the end of the second antennas coax.  Obviously, it depends a 
huge amount on the power you intend to run.  5W may safe.

If the second antenna is clearly in the far-field of the first, the 
coupled power is much much lower.  The signal will be huge in the second 
receiver, but not likely to be damaging to components.

I'm on the crew that activates Alpine County in the Calif QSO Party [no 
permanent hams there].  First year we tried M/2, we had CW and SSB in 
one cabin.  Considering that the aggregate ham experience of the crew 
was well over 200 years, I can't explain why we did this. :-)

The 80 and 75 inverted Vee's were undoubtedly in each other's 
near-fields, not sure about the 40's, but we had bandpass filters for 
each rig.  I was on 80 CW in the middle of the night, the phone op 
decided to try 75 ... he switched the filters, I called CQ, and I fried 
the diodes in the front end of his K3.  We now have the two stations at 
opposite ends of the campground [maybe 500 meters?], and we have no 
problems.

So George, I'd be very careful.  The cross-polarization would be in your 
favor but if you're on 20 or below and "close" means less than a 
wavelength at best, it could mean problems.

Hope this helps

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2014 Cal QSO Party 4-5 Oct 2014
- www.cqp.org