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Author Topic: Screwed up Screwdriver Grounding?  (Read 1784 times)
WA2ASB
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Posts: 39




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« on: June 25, 2013, 08:36:11 PM »

At Ham-Com in Plano this month I purchased a Tarheel 200A-HP from WB0W.  They didn't have one in silver so I had to get one in black and figured it wouldn't look good on my Silver BMW Z8 so I would hide it in the backyard behind a desert willow and close to the house ground.

The house ground is next to the power meter and a common point for grounds.  Reading the cautions about a long path to ground (mainly for vehicle installation I think) and somewhere reading that several short 2' ground rods were meter than an 8', I decided to get a couple of 5/8" ground rods from Home Depot, cut them into 2' sections around the base of the antenna and bring them into a common point. 

Just a few feet away from the antenna is a heavy wire that goes into the ground to a copper plate so I tied to it too.  Approximately 2.5' into the ground is sandstone, and according to my geologist neighbor who looked at it when they dug my pool, has a high iron content.

The pool is shallow, odd shaped, and has a surface area over 1,000 sq ft.  The re-bar in the pool is 5/8" and at 8" intervals.  All of this is connected to the common ground near the antenna.

So, I connected my ground rods and then a 3' piece of 4 GA copper to the common ground. 

Let me insert here that the reason for the lightning arrestors is that the house was previously hit by lightning, we live in a sub-division called "Highland Meadows", and my roof is the highest on the street (which accounts for the noisy mocking bird on my chimney for the fireplace).  Although I had minimal damage from the strike, a neighbor two block down burned to the ground.  That was what convinced me to have lightning arrestors installed.

Thank you for staying with me for the background information.  Now for the problem:

On the higher bands everything is fine.  However, on 80 meters I can only get a SWR reading of 3.0.  Just to see what it would do, I unplugged the ground completely and the SWR went to 4.0.  I'm assuming that it is a grounding problem, but I'm reluctant to disconnect from the common ground because I really don't understand all this stuff about groundings.  (I'm also trying not to tic God off so I don't get hit with lightning again.  I've been good, except for the slightly lustful look at a waitress this morning).

Oh, I left out the fact that I'm using an AA-230 to measure the SWR.  On the hight bands it shows XC, but on the lower bands it shows XL.  I really don't know how to interpret that.     

The Tarheel screwdriver is a stepping point to understand and establish a good ground.  Then it will be replaced by the StepIRR that is still in the box.

Any advice (keeping in mind the lightning [or waitress]) problem?
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KB3HG
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« Reply #1 on: June 25, 2013, 09:37:26 PM »

It may sound a bit off topic but I would contact the manufacturer and look at Al's site www.K0BG.com.

I can't picture your ground plane under the antenna, radials missing? matching transformer? swr of 3 sounds like about 17 ohms z. Xl would refer to long ,Xc would refer to being short.

look at the website, loaded with information

Tom Kb3hg
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W5FYI
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« Reply #2 on: June 25, 2013, 11:11:50 PM »

Short antennas are going to have a lower radiation resistance than a more efficient quarter-wave vertical. It wouldn't surprise me that your screwdriver antenna's 80-meter Rr would be less than 1-ohm, and probably closer to ½-ohm! For a 3:1 SWR, your other losses are in the vicinity of 16-ohms.

Another factor, any metal near the antenna will "suck" RF back to ground, and it sounds like your antenna is very close to the service entrance and service panel and its associated wiring, which are effectively detuning the antenna's loading coil.

In other words, the Tarheel needs to be out in the open, with a good radial field, for it to work best, and a higher SWR wouldn't hurt either if it is obtained by cutting resistive losses. The radial system is going to have the obstacles of the house on one side and the pool on the other, but any good radial field is better than none at all. I believe the inventor of the screwdriver antenna, Don Wallace, recommended an L-network at its feed point or, alternatively, a 9:1 unun or similar matching transformer.

Good luck.
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W5DXP
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« Reply #3 on: June 26, 2013, 06:06:49 AM »

However, on 80 meters I can only get a SWR reading of 3.0.

Expanding on w5fyi's first paragraph above.

Do you have a shunt matching coil (or cap) at the antenna feedpoint? If not, that is probably the problem. A resonant 80m screwdriver has a typical feedpoint impedance of around 17 ohms and of course, that is a minimum SWR of 3:1 at resonance. What I do is install a +j50 ohm shunt coil at the feedpoint. Then at the frequency where my feedpoint impedance is 25-j25 ohms, the +j50 shunt reactance will twist the feedpoint impedance to 50 ohms. Look up mobile shunt base matching coils (or caps) in The ARRL Antenna Book.

Here's the math: The parallel equivalent of 25-j25 ohms is 50 ohms of parallel resistance and -j50 ohms of parallel reactance. A parallel +j50 ohms of reactance neutralizes the -j50 ohms and results in a 50+j0 ohm feedpoint impedance.

A -j50 ohm shunt capacitor can be used. When the screwdriver is tuned for a feedpoint impedance of 50+j50 ohms, a -j50 ohm cap will result in a 50 ohm feedpoint impedance.
« Last Edit: June 26, 2013, 06:14:13 AM by W5DXP » Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
KA5IPF
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« Reply #4 on: June 26, 2013, 08:03:36 AM »

My Tarheel came with a shunt coil to mount at the base. Check your box and instructions.

Clif
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KB4QAA
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« Reply #5 on: June 26, 2013, 08:43:57 AM »

There are three kinds of grounds:

1. Electrical (safety)
2. Lightning
3. RF ground

You have installed an lightning ground.  Your shallow rock layer is not helpful, and the iron content is a moot point.   A 2.5ft rod is not better than an 8ft rod given average soil and the ability to install a full size rod.  Your approach of multiple short rods bonded together is the correct alternative.

As you have discovered, working an antenna against ground is a less effective method.  You need to install whatever radials are practical.  They need to attach to the the antenna/mount close to the feedline fitting.  Four radials would be the absolute minimum; eight will be better for stabilizing the SWR/impedance.  Ideal length, 1/4wl on the lowest band.

b.
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KE3WD
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« Reply #6 on: June 26, 2013, 02:02:59 PM »

The screwdriver is a Vertical Antenna. 

When mounted on a vehicle, it uses the metal mass of the vehicle as its counterpoise. 

When mounted as you are using it, it requires placing a Radial Field below it. 

Grounding with ground rods is good for lightning protection, but no good for use as a radial field for a vertical antenna on the ham bands. 

The good news is that you can easily make an array of ground radials underneath your screwdriver using just about any copper wire you can find, stranded or solid, insulated or uninsulated, even twisting and soldering different lengths together to make a longer one if necessary. 

The internet is full of howtos and pictorials on the subject of these ground radials and how to go about installing them, etc.  But you might do your websearch for "ground radials for vertical hf antennas" rather than limiting your searches to "screwdriver" which in most cases will likely be referencing mounting on the metal vehicle. 


73
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WA2ASB
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« Reply #7 on: June 26, 2013, 04:15:20 PM »

Thanks for all the insight and information.  Unfortunately with HMOA restrictions here, I'm somewhat limited to what I can do.  Also, after spending over $150K on the pool and landscaping, I don't want the XYL telling me what to do with the antenna.  (That reminds me: I have a colonoscopy scheduled for next week.  If any readers are over 50, get it done.  I have to have it every 5 years because of my brother's pre-cancerous polyps.)

I have a 21' SteppIR that I want to install as the permanent solution.  The 200A-HP will migrate to the SUV.  However, I installed it close to the house to test out the grounding.  Also, I wanted to see the effect of the ground rods and if I could put the Steppir close to the house.

When we had the pool dug, I had them make a berm behind the pool with the dirt they dug out.  We then had topsoil and plants put on the berm above the pond.  I could put the Steppir up on the berm.  It couldn't be viewed from the front of the house; the back and one side would be blocked by the weeping willow and vitex.  Only one neighbor could actually see it.

With it on the berm, I could run a lot of radials, but couldn't run very far in the direction of the pool (the pool is U-shaped and the pond is above it on the berm.  You can see it on google-earth.)  The downside would be the long coax run around the pool and into the house.  It would also have to be buried part of the way.

The Steppir has the 40 meter coil and the balun.  I'd also have to get much longer wire to control it too.

My wife is coming home for 2 weeks so I've got time to mull things over because while she is home she will keep me busy (all the roses have to be dug up and destroyed because of rosette virus).
 
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WB6BYU
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« Reply #8 on: June 26, 2013, 04:28:03 PM »

Adding radials will help efficiency a lot, though they may make the SWR worse on 80m.
That's a fact of life with short verticals:  they have a low feedpoint impedance, so you have
to keep the ground loss low to maintain efficiency.

Run as many as you can as far as you can, to a reasonable distance.  They don't all have
to be equal.  I'd aim for 16 wires at least:  if you can only run 5' or 10' in some direction,
do it.  If you can go 30, great.  You can just lay them out on top of the ground (at least
while you wife is gone) to see how much difference it makes.

Put the matching coil or transformer at the base of the antenna to improve the SWR on
80m and 40m.  While a capacitor will also work, the coil can be selected for 80m and
then left in place on the higher bands:  it will have less effect as you go up in frequency
where the SWR is better anyway.

Ground rods are generally a poor ground for RF, because dirt isn't a good conductor, and
the skin effect keeps most of the RF currents more towards the surface (which is why
long ground rods aren't any better for RF even though they may provide a better
lightning ground.)
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K7JQ
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« Reply #9 on: June 26, 2013, 08:12:59 PM »

I use a ground-mounted Tarheel 200A-HP in an HOA restricted community in Arizona. Fortunately, I have it mounted high up on a hill behind my house, hidden by desert brush, so no one can see it. Check out pictures of it on my QRZ.com page. I have it mounted 4 inches off the ground, with sixty, 25-foot radials (they don't have to be resonant) arranged in a spoke-like fashion stapled to the ground, and use the supplied shunt coil to match it on 40 and 80 meters (read the instruction manual on its' adjustment). SWR from 10 thru 80 meters is always 1.5:1 or less. Understand that this is about as optimum a situation as you can get with a short, compromised vertical....nowhere near any metallic objects or structures, no RFI anywhere to anyone on transmit, no electrical household noise on receive, and it can "see" for miles in all directions with no obstructions. I run a KW, and have worked over 260 countries with it in the past 6 years, so it can be an effective radiator when installed properly.

That being said, obviously its' performance will vary depending on how close it's mounted to objects, and the availability of room for a sufficient radial system. You have to do the best you can with what you have to work with. By the way, you mentioned a "long" run of coax and control cable to your proposed Steppir vertical. My run of LMR400 coax and motor control line is 450 feet, buried all the way down the hill to keep the critters off of it. I'm sure there's (unpreventable) loss, but it still works fine. I doubt if your run will be anywhere near that long :>) Good luck with your project!

73, Bob  K7JQ     
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KG4RUL
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« Reply #10 on: June 27, 2013, 06:36:44 AM »

The SteppIR will also need a radial field to operate properly.
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WA2ASB
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« Reply #11 on: July 02, 2013, 08:18:44 PM »

Dennis, that is true about the Steppir, and I ordered 3 sets from them, and based on the advice from the Elmers above, I'm going to install it on the berm up above the pool and pond.  I'll bite the bullet on the long coax run.  I've just got a lot to do before I get around to it.  One of the 3 skimmers has a break in the line and I have to find it.

Fortunately, I convinced the XYL that I have more to do here in the house and yard that I can't come back to India until 4th quarter. 

Our apartment is on the 5th floor of the Leela Kempenski and rather than using "Hamsphere", I'm thinking about applying for the ability to run remote from there, but it would be QRP.
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