开云体育

I saw a video about the dangers of using the nanoVNA by connecting to the antenna coax: static buildup on the coax. So I guess one way around this is to measure right at the antenna. The nano is portable. But that still leaves the coax so I suppose that I should short out the coax before measuring it. Any other measures/precautions about measuring out doors antennas?
syd/wt1v

When connecting any length of coax to any sensitive measurement instrument,
I ALWAYS first grab the end of the coax and effectively short the end with
a finger or hand. If there has been a recent lightning storm, you might be
surprised and the discharge might surprise you in an undesirable manner.
The cylindrical capacitor that makes up the coaxial cable can hold a charge
for a very long time.

Dave - W?LEV

On Fri, Aug 13, 2021 at 3:06 PM Syd via groups.io <nhuq1@...>
wrote:

I saw a video about the dangers of using the nanoVNA by connecting to the
antenna coax: static buildup on the coax. So I guess one way around this
is to measure right at the antenna. The nano is portable. But that still
leaves the coax so I suppose that I should short out the coax before
measuring it. Any other measures/precautions about measuring out doors
antennas?
syd/wt1v

--
*Dave - W?LEV*
*Just Let Darwin Work*

On 8/13/21 8:06 AM, Syd via groups.io wrote:

I saw a video about the dangers of using the nanoVNA by connecting to the antenna coax: static buildup on the coax. So I guess one way around this is to measure right at the antenna. The nano is portable. But that still leaves the coax so I suppose that I should short out the coax before measuring it. Any other measures/precautions about measuring out doors antennas?
syd/wt1v

Static will build up on the antenna, not just the coax. Or even the stuff sitting on the bench.? The ESD sensitivity varies substantially among the various small VNAs. Some have resistor bridges/pads right on the inputs.? Others have MMIC switches. The latter are more sensitive.

The other thing to worry about is nearby transmitters inducing sufficient power into your antenna to cause problems.


I suppose, one could check for any of these by measuring the AC voltage across a 50 ohm load hooked up to the UUT, with a simple (cheap, sacrificeable) diode probe. A 1N4001 and 0.01 uF would probably work, since you're looking for "volts" not "microvolts". A $10 DMM, diode, etc.

On 8/13/21 8:12 AM, David Eckhardt wrote:

When connecting any length of coax to any sensitive measurement instrument,
I ALWAYS first grab the end of the coax and effectively short the end with
a finger or hand. If there has been a recent lightning storm, you might be
surprised and the discharge might surprise you in an undesirable manner.
The cylindrical capacitor that makes up the coaxial cable can hold a charge
for a very long time.

Blowing dust, snow, or rain, are even more of a charging hazard than the field from a passing thunderstorm.? Continuous charging. It charges until something breaks down, so you get a buzz or continuous clicking. A sort of relaxation oscillator.

Yep. One day long ago I was in my shack in the evening and I heard "snap, snap, snap, snap..." and wondered what the heck? I quickly found the source. I had a feedline's PL259 disconnected and dangling free. Every half a second there was a bright 2" arc from the feedline to a nearby grounded surface. It had started to snow.

I developed the habit long ago of shorting out any signal connectors before mating them when working at the bench or when connecting antennas. As others have mentioned, I usually just just use my finger between the center and shield while touching the ground of the device I'll be connecting it to. All my antennas in the shack go through switches which keep the antenna connected to ground until I switch it to a radio or analyzer. I also usually have Polyphasor or similar in line.

I use my wedding ring to short the coax... Or make sure I touch the coax shield and center... You can still get a static discharge from the antenna at the base.

73, and thanks,
Dave (NK7Z)

ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

As a matter of routine, consideration should be given to installing a

bleeder across the antenna's lead in the station to drain static
accumulation and never mind if it's a "grounded" antenna. A permanent
bleeder like that does not affect reception or transmission. For the
present purpose it would not affect v.n.a. observations. In urban areas
particularly, diode and gas discharge devices can cause problems; old
fashioned "R" is all that's required.

John

at radio station VE7AOV.

+++++


On 2021-08-13 8:27 a.m., Jim Lux wrote:

On 8/13/21 8:12 AM, David Eckhardt wrote:

When connecting any length of coax to any sensitive measurement
instrument,
I ALWAYS first grab the end of the coax and effectively short the end
with
a finger or hand.? If there has been a recent lightning storm, you
might be
surprised and the discharge might surprise you in an undesirable manner.
The cylindrical capacitor that makes up the coaxial cable can hold a
charge
for a very long time.

Blowing dust, snow, or rain, are even more of a charging hazard than
the field from a passing thunderstorm.? Continuous charging. It
charges until something breaks down, so you get a buzz or continuous
clicking. A sort of relaxation oscillator.

This is a video of what voltages an antenna can present to equipment. Poor video, but worth watching.

73, Joe, K1ike

Just a quick question. How many ohms would you suggest for that bleeder?

Fred - N4CLA

On Fri, Aug 13, 2021 at 1:42 PM John Nightingale via groups.io <if455kc=
[email protected]> wrote:

As a matter of routine, consideration should be given to installing a

bleeder across the antenna's lead in the station to drain static
accumulation and never mind if it's a "grounded" antenna. A permanent
bleeder like that does not affect reception or transmission. For the
present purpose it would not affect v.n.a. observations. In urban areas
particularly, diode and gas discharge devices can cause problems; old
fashioned "R" is all that's required.

John

at radio station VE7AOV.

+++++


On 2021-08-13 8:27 a.m., Jim Lux wrote:

On 8/13/21 8:12 AM, David Eckhardt wrote:

When connecting any length of coax to any sensitive measurement
instrument,
I ALWAYS first grab the end of the coax and effectively short the end
with
a finger or hand. If there has been a recent lightning storm, you
might be
surprised and the discharge might surprise you in an undesirable manner.
The cylindrical capacitor that makes up the coaxial cable can hold a
charge
for a very long time.

Blowing dust, snow, or rain, are even more of a charging hazard than
the field from a passing thunderstorm. Continuous charging. It
charges until something breaks down, so you get a buzz or continuous
clicking. A sort of relaxation oscillator.

I have a 1 MΩ permanently connected across the output of my transmatch.
When the feedline is disconnected from the transmatch (most of the time), I
have a 50 Ω resistor paralleled with a 70 volt breakdown spark gap. Both
are 5-watt resistors.

Dave - W?LEV

--
*Dave - W?LEV*
*Just Let Darwin Work*

On 8/13/21 1:03 PM, Fred Moore wrote:

Just a quick question. How many ohms would you suggest for that bleeder?

Megohms.

I used to use scrap black vacuum hose - it was UV resistant and happened to be conductive, and best, it was cheap.


Now, I'd probably just use a 1 Meg resistor if I had one.

Fred - N4CLA

On Fri, Aug 13, 2021 at 1:42 PM John Nightingale via groups.io <if455kc=
[email protected]> wrote:

As a matter of routine, consideration should be given to installing a

bleeder across the antenna's lead in the station to drain static
accumulation and never mind if it's a "grounded" antenna. A permanent
bleeder like that does not affect reception or transmission. For the
present purpose it would not affect v.n.a. observations. In urban areas
particularly, diode and gas discharge devices can cause problems; old
fashioned "R" is all that's required.

John
at radio station VE7AOV.

+++++


On 2021-08-13 8:27 a.m., Jim Lux wrote:

On 8/13/21 8:12 AM, David Eckhardt wrote:

When connecting any length of coax to any sensitive measurement
instrument,
I ALWAYS first grab the end of the coax and effectively short the end
with
a finger or hand. If there has been a recent lightning storm, you
might be
surprised and the discharge might surprise you in an undesirable manner.
The cylindrical capacitor that makes up the coaxial cable can hold a
charge
for a very long time.

Blowing dust, snow, or rain, are even more of a charging hazard than
the field from a passing thunderstorm. Continuous charging. It
charges until something breaks down, so you get a buzz or continuous
clicking. A sort of relaxation oscillator.

I always connect my coax from all antennas to ground when not in use. No problems with static then and I can measure them any time I need/want to. Well, as long as I've just disconnected them from their normal grounded resting place.

--
Phil KE3FL

I would always discharge an antenna coax before measuring, just to be on the safe side. However, I would assume that if one has an unun, balun, or matching transformer between the antenna an receiver end of the coax the antenna already has a path to ground and is in all likelihood self-discharging. Please correct me if I'm wrong, but this is something to consider. Safest practice would be to short the coax regardless.? 73
Ken --? WB?OCV

No, not necessarily. Some types of these do not short both conductors together and even if they do, there's no guarantee that the potential of the feedline will be the same as that of your device until they have been equalized. The feedline and antenna may all be at ground potential, but you and your device may have accumulated a static charge, which should be dissipated before making the connection.

Fred: You would need a resistor which would handle the power AND the voltage it would see; for legal limit transmit power, many otherwise-suitable resistors have insufficient voltage rating. Phil AD5X uses one or more 3MΩ high-voltage resistors:

You could also use a choke, Phil shows the construction of a legal-limit HF bias tee, the choke in it should also do the job:
For less than legal limit power much less robust (and cheaper) components could be used. Of course chokes are frequency-sensitive unlike resistors.

Bleeders are only required for antennas which do not have a DC return between center and outer conductors; many dipoles or verticals with baluns or transformers, folded dipoles, Yagis with folded dipole feeds or hairpin match, etc. already have a DC return inherent in their designs; a quick measurement with an ohmmeter will tell. If it shows a low resistance a bleeder is unnecessary.

I also use gas-discharge tube arrestors on ALL of my antenna feedlines and rotor control lines, with the gas tubes sized for the power levels to be used; in my station, receive only, rotor(56V), 100W (150V), or 500W (1000V). My arrestors, which have replaceable/swappable gas tubes, are mounted directly to one of my 4 station ground rods using DX engineering mounting clamps: Replacement gas tubes are available inexpensively from distributors like Digi-Key. I mark the arrestor with the installed gas tube's voltage for easy reference.
Here is a QEX article with more information:

My station has already withstood a nearby lightning strike, unlike my fiber internet modem/router and everything on my wired network, including some of the wires!

The arrestors are for lightning protection only, they will NOT bleed down to a low-enough voltage to protect a network analyzer input!
Even with bleeders installed it is good practice to short out any cable before attaching it to an instrument.

73, Don N2VGU

On 8/13/21 3:35 PM, Jim Lux wrote:

On 8/13/21 1:03 PM, Fred Moore wrote:

Just a quick question.? How many ohms would you suggest for that bleeder?

Megohms.

I used to use scrap black vacuum hose - it was UV resistant and happened to be conductive, and best, it was cheap.

Also low inductance, low capacitance.


Kent, AC1HJ

Jim Lux and I have both written 1 MegOhm (if Jim had such a resistor). Not
that this an optimum value by any means. It's just what we had. The sole
purpose of the resistor is to keep voltage from building up on the coaxial
(or parallel conductor ) transmission line. A dipole offers an open
circuit for DC voltages. With differential voltages from where ever in
nature (rain, dust, snow, wind, lightning,.....) applied across the two
conductors of the dipole or any other open circuit radiating structure,
voltages will build up on the transmission line. The resistor across the
feedline in the shack serves the same function as the bleeder resistors
across the filter capacitors of amateur power amplifiers DC supplies I
remember my Novice transmitter, the Heathkit DX-40. It had a large-valued
RF choke from the back of the PL-259 to chassis. The sole purpose of that
RF choke was to keep voltages from building up on the feedline - same as
the resistor in question.

As such, the resistor can be inductive. It doesn't matter as its' dealing
with DC, so the complex portion of the impedance is zero. The only
consideration I'd throw out is that it have adequate power dissipation to
not involve itself in loading the source impedance of the shack end of the
feedline. Also, that it be high enough not to potentially dissipate the
power applied to the shack end of the feedline. Both those statements
imply the same consideration. 100 k to a couple of megohms is good.

Dave - W?LEV

--
*Dave - W?LEV*
*Just Let Darwin Work*

Dave: That choke also serves another very important function; it will short the plate supply to ground if the anode coupling capacitor should suffer from leakage current or fail short, keeping you from impressing plate voltage onto your antenna and whatever else is attached to the output terminal.
73, Don N2VGU.

One useful solution is to use a shunt resistor or a shunt inductor. Many antenna installations will include a resistor or a choke from the "hot" side of the coax to ground. The impedance of the shunt element is selected to be high enough that it's effectively "invisible" to the frequencies of interest, but low enough that it allows (high-voltage, low-current) static electricity to drain away safely to ground.

If, for example, you want to measure a typical 50-ohm-nominal antenna system which has BNC or "UHF" connectors, buy yourself a "T" adapter for the connector type you use, and buy a spare male plug of that sort as well. Solder a 10k or 100k-ohm resistor into the spare plug, and (at the same time) solder a length of wire and a ground clip to the "shell" side of the connector. When you want to measure an antenna, first clip the ground wire of the T adapter to a good ground, and then connect the antenna wire to one arm of the T. This will ground the coax braid, and allow any static buildup on the inner conductor to drain away through the drain resistor. Then, connect the other arm of the "T" to your measurement device (e.g. nanoVNA). 100k in parallel with the actual feed-point impedance won't make a measurable difference in what you read.

This is one instance in which old-style "carbon composition" resistors are very well suited - they're non-inductive, and they tend to be tolerant of high-voltage spikes. The fact that they're often noisy and "drifty" matters not at all in this application.

A 2-watt carbon composition resistor has a rated breakdown of only 2 kV.
Inductance really does not matter in this application as we're dealing with
DC and slow bleed of possible accumulating voltages. The complex portion
of the complete impedance expression does not exist for DC. The ±j term
only applies to AC, not DC.

Another application of the "bleed resistor" or inductor: I've lost MMIC
preamps and LNA's due to close-by lightning strikes. These strikes
notoriously induce intense close-by RF fields which can permanently damage
these MMICs. Once I lost enough of them, I started installing a 1 k or 10
k resistor across the input connector with as short of leads as possible,
usually effectively leadless. Since I've made that a practice on my home
brew MMIC LNAs, I've not lost a single one. Even the J-310 series is
susceptible to this failure and can easily be treated with the shunt
resistor. I prefer using the resistor rather than the inductor. Manty
inductors can introduce hits on the noise figure of the MMICs. Resistors
generally when installed properly do not exhibit resonances anywhere near
the frequencies of interest where inductors may.

Dave - W?LEV

On Mon, Aug 16, 2021 at 9:39 PM David Platt <dplatt-groups@...>
wrote:

One useful solution is to use a shunt resistor or a shunt inductor. Many
antenna installations will include a resistor or a choke from the "hot"
side of the coax to ground. The impedance of the shunt element is selected
to be high enough that it's effectively "invisible" to the frequencies of
interest, but low enough that it allows (high-voltage, low-current) static
electricity to drain away safely to ground.

If, for example, you want to measure a typical 50-ohm-nominal antenna
system which has BNC or "UHF" connectors, buy yourself a "T" adapter for
the connector type you use, and buy a spare male plug of that sort as
well. Solder a 10k or 100k-ohm resistor into the spare plug, and (at the
same time) solder a length of wire and a ground clip to the "shell" side of
the connector. When you want to measure an antenna, first clip the ground
wire of the T adapter to a good ground, and then connect the antenna wire
to one arm of the T. This will ground the coax braid, and allow any static
buildup on the inner conductor to drain away through the drain resistor.
Then, connect the other arm of the "T" to your measurement device (e.g.
nanoVNA). 100k in parallel with the actual feed-point impedance won't make
a measurable difference in what you read.

This is one instance in which old-style "carbon composition" resistors are
very well suited - they're non-inductive, and they tend to be tolerant of
high-voltage spikes. The fact that they're often noisy and "drifty"
matters not at all in this application.

--
*Dave - W?LEV*
*Just Let Darwin Work*