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RF Ground #14606 09/25/02 04:38 AM
Joined: Jul 2002
Posts: 8,262
Trumpy Offline OP
Member
How do you achieve a true RF ground at the base of a steel tower,with a concrete pad,
does it entail using a large amount of copper as a grid?.
What is required, as I am used to installing power systems earthing, but at RF frequencies these types of earthing systems can actually reduce the MUF.
Can someone please help?.

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Re: RF Ground #14607 09/25/02 02:24 PM
Joined: May 2001
Posts: 717
G
George Corron Offline
Member
Trumpy,
Sorry, saw your post early this morning, just forgot to answer.

Most towers have a driven ground rod system, and that part MUST be tied to the system grounded conductor for safety.

Yup, you've gotta be very careful about de-tuning your antenna, but if you run straight down the tower, that should not be a problem.

You do have to be careful about running into a radial system, should that be required that you can detune pretty quickly if tied together improperly, most people do, but it does most certainly affect the performance, and never for the better.

Long story short, get the guy who designed the antenna to check if your into a detuning stage, or gimme the freq, and length or tower and I'll try to help (though for accuracy, it may take me a day). Some guys who design these things really never check it cause they don't know.

Re: RF Ground #14608 09/25/02 02:40 PM
Joined: Aug 2001
Posts: 7,520
P
pauluk Offline
Member
Yep, straight down the mast to the ground in order to minimize inductance in the lead. As the frequency goes up, so it becomes more critical. The worst condition would be if the stray inductance and stray capacitance happen to form a parallel resonant circuit at the frequency in use. The ground connection would then act as a high impedance at that frequency.

Re: RF Ground #14609 09/25/02 05:01 PM
Joined: Nov 2001
Posts: 280
M
motor-T Offline
Member
Trumpy brought up the grounding of antennas, what about matching. Does body use Smith-Charts anymore?

mark

Re: RF Ground #14610 09/30/02 02:21 AM
Joined: Jul 2002
Posts: 8,262
Trumpy Offline OP
Member
The tower in question is actually used for a Citizens Band cross-polarised 4 element Yagi,
the tower is 14 metres high, with a stub-shaft extending another 4 metres above the top of the tower proper,(for the rotator).
Beam is mounted in middle of stub-shaft.
Centre frequency is 26.570MHz.
Just a short note, can anyone tell me of a practical way of making a Gamma-Match unit, if you cannot get a hold of a proper air-type variable capacitor, to effect the coarse tuning of the Match?. [Linked Image]

Re: RF Ground #14611 09/30/02 07:33 PM
Joined: May 2001
Posts: 160
C
Chris Rudolph Offline
Member
What is a Gamma match?Is it a broadband matching circuit that requires no variable tunning elements?
Where is the Gamma match placed in the circiut?Is it between the 50 ohm transmission line and the feed element to the antenna?I assume that it will match the transmission line to the input impedance of the antenna assuming of course that the complex input impedance is known.Finding or measuring this impedance might be tricky,because as you pointed out it is a strong function of the grounding
counterpoise.
Chris

Re: RF Ground #14612 10/10/02 01:33 AM
Joined: Jul 2002
Posts: 8,262
Trumpy Offline OP
Member
Thanks for your input Chris,
this job is really proving to be a head-ache for me,the guy had the tower and aerials erected and then thought about wiring them, he hooked the coax straight on to the aerials, the tower has no earthing, not even for lightning protection, and yet he rings up my boss and tells him I'm not doing my job properly.
CBers eh?.

Re: RF Ground #14613 10/10/02 02:29 PM
Joined: Aug 2001
Posts: 7,520
P
pauluk Offline
Member
The gamma-match involves tapping the hot feed from the coax at the right distance from the center point to properly match the impedance. (Named for the rought resemblance to the Greek letter gamma, in the same way as we have pi-networks, delta xfmrs, etc.).

Quote
CBers eh?.

I did service and repair on CB equipment a good few years ago. Some of the people I had to deal with were unbelievable. Ignorance of technical facts I can live with (hey we're not all experts in everything!), but the most annoying were the ones who were convinced they were always right, even when totally wrong. ("My rig puts out 50 watts now," even though the DC input was only 10W, for example!)

Re: RF Ground #14614 10/10/02 03:01 PM
Joined: May 2001
Posts: 160
C
Chris Rudolph Offline
Member
Once you have properly grounded the tower,put a thru line watt meter at the output of the transmitter(50 ohms)and measure the forward power and the reflected power.Hopefully the reflected power will be less than 10% of the forward power(over the frequency range) indicating an adequate match between the transmission line and the antenna .
Let me know what happens.
Chris

Re: RF Ground #14615 10/12/02 11:51 PM
Joined: Jul 2002
Posts: 8,262
Trumpy Offline OP
Member
Chris,
Had to go out to the aerial connections on the end of a crane hook, wearing a 7 point
safety harness,fixed up all of the bad connections, installed a Gamma rod, no capacitor, just solid aluminium rod, took two more trips up to the aerial, to get an
SWR of 1.15:1, I was impressed, and finished the job(Thank Christ!).
I am a radio ham as well as a CBer, but when I put up my tower, I had the sense to actually do things in a logical sequence.


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