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Edward R Cole

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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

----------------------------
From: Ralf Wilhelm <[hidden email]>
Cc: "[hidden email]" <[hidden email]>
Subject: Re: [Elecraft] Antenna question
Message-ID:
<[hidden email]>
Content-Type: text/plain; charset=us-ascii

Hi George,

The 130 feet corresponds to lambda/2 on 80 and you can use the far
field approximation (that Ed is using) there...

At short distances (less than a quarter or a sixth of the
wavelength), however, 1/r^2 and 1/r^3 ("near field") components of E
and H fields are still present (or dominant) and the far field
approximation should not be used. You also have to be careful with
the cross-polarization argument, since the electrical near field has
all three vector components almost anywhere in space and the coupling
can be much higher (depending on how well symmetry is preserved in
the "yagi+vertical system").

Better use a NEC based program (e.g. EZNEC or the free 4nec2), if you
have on access to a milliwatt-meter/scope and have to calculate...

By the way, P should read
P= E*I = E^2/R
=> E=sqrt(P*R)=223mV
for a 0dBm (S9+67dB) signal (?)

Greetings

Ralf, DL6OAP

73, Ed - KL7UW
http://www.kl7uw.com
"Kits made by KL7UW"
Dubus Mag business:
[hidden email]

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DL6OAP

Re: Antenna question

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 d
rastically 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 <[hidden email]>:

> 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
>
>

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Bill Turner

Re: Antenna question

I see a number of people are trying to calculate whether any damage
would be done by the situation originally posted.

Frankly, I think this is a dangerous approach. There are too many
variables in a particular situation to risk depending on calculations
when expensive equipment is endangered.

If you must go ahead with your situation, I strongly suggest, as others
have done, that you measure the interaction rather than try to calculate
it.

73, Bill W6WRT

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k6dgw

Re: Polarization loss [was "no subject"]

In reply to this post by Edward R Cole

On 2/12/2014 12:46 PM, Edward R Cole wrote:
> Ralf,
>
> Thanks for catching my mistake in the ohm's law formula.
> P = E^2/R

It's been at least 100 years since I worked antenna problems out on my
K&E log-log duplex vector slide rule, but the rule of "20 dB loss with
crossed polarization" doesn't apply well [or at all] in the near-field.
And, the near-field doesn't just suddenly end, it declines as 1/d^k
where k is usually somewhere between 2 and 3 depending on a bunch of
other factors. The result is that the NF fades into the FF, there's no
sign out from the antenna that says, "The Far-Field starts here."** :-)
Again, I would be extremely careful connecting a second antenna to a
receiver if the antennas are at all close. Many a radio has been fried
on Field Days.

On the subject of destroying the front end of a receiver, be very wary
of precipitation static with ungrounded antennas. Each rain drop or
snowflake deposits a tiny charge. In the absence of some path to
ground, each tiny charge will add to the total charge on the input
capacitance of the first stage [like an FET], and eventually the voltage
will rise high enough to destroy the device. I believe Elecraft radios
have internal bleeds on the antennas but I still have external ones on mine.

The same crew with over 200 hours of aggregate ham experience that put
the CW and SSB stations in the same cabin watched a tiny little bit of
"grass" on the baseline of a 756Pro2 that we couldn't even hear as it
began to snow. Then silence. So, being highly experienced, we replaced
it with another 756Pro2, and watched it die the same death. Easiest way
to do it externally is to solder a 50K or so resistor into a PL-259,
screw it onto one arm of a coax T, put the antenna on the other arm and
screw the T onto the radio. You can use an RF choke instead of a
resistor, however that can sometimes lead to parasitic resonances.

73,

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

**The original transcontinental railroad began in Sacramento. The
government subsidy to the railroad was higher for construction in the
mountains. The terrain around Sacramento is very flat, and the
foothills of the Sierra Nevada are a good 30 miles or more to the east.
A few miles NE of downtown is a golf course [flat], and there is [or
was] a plaque on a rock that read, "The mountains begin here."

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Charlie T, K3ICH

Re: Antenna question

In reply to this post by Bill Turner

Wire a small "grain-of-wheat" incandescent light bulb to the unused antenna
feed and fire up the transmitter. If the bulb lights brightly, you need
some protection. Admittedly, this is a crude test, but it can point to a
problem and takes into account all those variables in the calculations. Try
all bands and start at low power, otherwise, you may blow the bulb before
any relative results can be gleaned.

73, Charlie k3ICH

----- Original Message -----
From: "Bill Turner" <[hidden email]>
To: <[hidden email]>
Sent: Wednesday, February 12, 2014 5:45 PM
Subject: Re: [Elecraft] Antenna question

>I see a number of people are trying to calculate whether any damage would
>be done by the situation originally posted.
>
> Frankly, I think this is a dangerous approach. There are too many
> variables in a particular situation to risk depending on calculations
> when expensive equipment is endangered.
>
> If you must go ahead with your situation, I strongly suggest, as others
> have done, that you measure the interaction rather than try to calculate
> it.
>
> 73, Bill W6WRT
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

Phil Wheeler-2

Re: Antenna question

In reply to this post by Bill Turner

Bill,

You don't trust the traditional "Smoke Test"? :-)

73, Phil w7ox

On 2/12/14, 2:45 PM, Bill Turner wrote:

> I see a number of people are trying to calculate
> whether any damage would be done by the
> situation originally posted.
>
> Frankly, I think this is a dangerous approach.
> There are too many variables in a particular
> situation to risk depending on calculations
> when expensive equipment is endangered.
>
> If you must go ahead with your situation, I
> strongly suggest, as others have done, that you
> measure the interaction rather than try to
> calculate it.
>
> 73, Bill W6WRT

Don Wilhelm-4

Re: Polarization loss [was "no subject"]

In reply to this post by k6dgw

A 'technical' consideration - rather than a creating a path to 'ground'
for the antenna, it is better to create a DC path across each feedline.
Fred's suggestion works and works quite nicely.

73,
Don W3FPR

On 2/12/2014 6:09 PM, Fred Jensen wrote:
> Easiest way to do it externally is to solder a 50K or so resistor into
> a PL-259, screw it onto one arm of a coax T, put the antenna on the
> other arm and screw the T onto the radio. You can use an RF choke
> instead of a resistor, however that can sometimes lead to parasitic
> resonances.
>

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EricJ-2

Re: Antenna question

In reply to this post by Phil Wheeler-2

Surprised me too. It ALWAYS produces results.

Eric
KE6US

On 2/12/2014 3:46 PM, Phil Wheeler wrote:

> Bill,
>
> You don't trust the traditional "Smoke Test"? :-)
>
> 73, Phil w7ox
>
> On 2/12/14, 2:45 PM, Bill Turner wrote:
>> I see a number of people are trying to calculate whether any damage
>> would be done by the situation originally posted.
>>
>> Frankly, I think this is a dangerous approach. There are too many
>> variables in a particular situation to risk depending on
>> calculations when expensive equipment is endangered.
>>
>> If you must go ahead with your situation, I strongly suggest, as
>> others have done, that you measure the interaction rather than try to
>> calculate it.
>>
>> 73, Bill W6WRT
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>

Bill Turner

Re: Antenna question

In reply to this post by Phil Wheeler-2

ORIGINAL MESSAGE: (may be snipped)

On 2/12/2014 3:46 PM, Phil Wheeler wrote:
> Bill,
>
> You don't trust the traditional "Smoke Test"? :-)
>
> 73, Phil w7ox

REPLY:

Of course I trust it. I use it whenever I need smoke. :-)

73, Bill W6WRT

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