Rangemaster with ground radials

I have had the opportunity to use several different ground radial configurations with my Rangemaster over the years. In all the installations the transmitter was mounted on a iron pipe at least ten feet in the air. In all cases, the"safety ground" lead (12 gauge insulated) to the 4-8 foot ground rod, was run down the pipe on old television twin-lead style standoffs. The pipe was not connected to the ground terminal on the transmitter. The ground radial system(s) were connected to the bottom of the safety ground lead at the ground rod. The reasoning to the use of ground radials is to provide a low resistance path for an electrical potential to discharge.

The minimum number of ground radials used was 4 and the maximum was 16. The radial conductor length was about 20 feet. Additional length didn't seem to perceptibly increase "listen-ability". The transmitter, operating with just a ground rod, consistently could be heard out to about 100 yards without objectionable noise. The more radials, the greater the distance. The radial systems were spaced evenly 360-degrees around the ground rod, laid tight on the ground with metal clips and not buried. 16 radials increased the signal to 3/4 of a mile without objectionable noise. I can image that 32 ground radials would significantly increase the signal in comparison to just four radials. The transmitter required re-tuning after each change in the ground system. More radials did seem to cause the tuning dip to be sharper. Terrain, buildings and obstructions did punch holes in the signal area and shorten range.

It is important to isolate the mount pipe from the transmitter ground to keep it from radiating. The safety ground conductor should be allowed by inspectors just because its good engineering practice. Its in the rules. The safety ground conductor connects directly to the ground terminal on the transmitter. Don't forget the standoffs. This arrangement will allow you to put a choke and/or ferrite beads (filter) on the conductor if the inspector requires it.

The ground radials can be as small as 20 gauge insulated copper bell wire. The radials carry very little current and a surprisingly high voltage (100-200 volts) because of their high impedance when the system is properly tuned. Results from your own installation may vary from mine.

Marshall,

Your report on the use of radials is interesting and should be useful to those contemplating an installation. It did, however, raise a couple of questions.

It is important to isolate the mount pipe from the transmitter ground to keep it from radiating.

According to the National Electric Code antenna masts must be grounded. Was it? Our local code also requires either bare #8 or conduit protected #10 wire for antenna grounding. Though smaller wire works for operating grounds (no choke) the large wire is required for protection.

The radials carry very little current and a surprisingly high voltage (100-200 volts) because of their high impedance when the system is properly tuned.

This is surprising. Between what points was this potential measured? I can see a potential would exist along the radials if they are a significant fraction of a wavelength in length but for short radials there should be almost no potential along or between the radials. Could you clarify this?

Thanks,

Neil

I have a Talking House transmitter and use the Talking House remote antenna ATU. The antenna/ATU is mounted above my TV antenna which is on a 5 foot mast on a tripod on the 2nd floor roof.

From the ATU ground connection, a 2 foot ground wire connects to 6 radials, each about 12 to 15 feet long. They are equally spaced around the base of the antenna.

I found there was an improvement in coverage. Not a substantial difference but a noticable difference.

Neil,

1) The FCC could give a rip about the NEC codes. This I know from experience. If it radiates and it goes substantially beyond the Part 15 3 meter limit for length, there is no joy. Electrical inspectors know absolutely nothing about tuned RF; and have no motivation to learn (FCC vs NEC). For a licensed station with 10 watts or more, all of the NEC code parameters you point to come into play. For Part 15, not so much. The base of the mast can be connected to the ground rod, but the transmitter ground terminal must not use the mast as its ground conductor. The conductor between the base of the mast and the ground rod can be of appropriate material and size. I know, we all understand the mast will radiate some anyway, but this way the ground lead filter works better and the FCC inspector is happier.

2) Impedance distribution along a ground radial of any length (non-resonant) would show maximum reactance at the end of the radial as part of the vertical monopole antenna system. The same impedance swing occurs on the radiating antenna. This puts the maximum current lobe of the circuit nearest the feed point. Whether using a counterpoise ground or a radial system, the point farthest from the feed point has the highest impedance and therefore would have the peak voltage potential. The potential is expressed using the radiation resistance of the antenna system (Ohms Law). These potentials and voltages usually cannot be measured and are considered theoretical in nature; that is until an animal or kid gets a hold of the end of the live radial and suffers an Rf burn (caution).

Most basic antenna theory deals with resonant RF radiating systems. When dealing with non-resonant systems, the current and voltage lobes are less predictable and thus can be less than calculated or much higher than anticipated in real-world practice. Most short non-resonant antenna systems exhibit a very low resistance, high reactance, with "Q" values all over the place (usually low). More ground radials improve Q which usually produces the sharp dip/peak when tuning

All of this said, ground radials do improve the perceptible range of Part 15 AM BCB installations when care is taken to satisfy station owners, listeners and the FCC.

Marshall,

1) When I mentioned the NEC requirements regarding mast grounding I was not referring to the FCC rules since, as you know, the two address separate issues. The NEC and my local code grounding requirements have nothing to do with whether the mast is for supporting either a transmit or receive antenna. They apply to both purposes hence the power of the transmitter is irrelevant. Therefore, the NEC grounding requirements, if adopted by local regulators, apply to part 15 antenna masts and lead in wires. Let's not give the impression that part 15 operations are exempt from these requirements.

2) Impedance distribution. This approach to the functioning of very short ground radials is subject to the fact that if the radials are in close proximity to the earth there is capacitive coupling to the earth and current induced in the earth will partly pass through the radials. The earth is now part of the radial system and the "maximum reactance" point is not at the end of the radials. This, unlike the vertical radiator, is not a resonant system with essentially zero current at the end.

The reactance at the tip of an antenna as defined by the vector quotient V/I is high because the current is essentially zero. This is not the case for radials due to the coupling to the earth and the voltage at the radial ends would be much lower if measured with respect to ground as compared with the antenna voltage.

I am still pondering the origin of your "100-200 volt" numbers.

Neil

Talking about ground radials necessarily combines with questions about safety ground, and right this moment I came up with a plan.

Whereas we part15ers tend to DIY, I believe I will call in a professional once I install an outdoor antenna structure, namely a licensed electrician.

His assignment will be to install a safety ground at the transmitter according to all codes and standards, with no regard whatsoever (in his assignment) to any question of RF.

Once this professional safety ground arrangement exists, with full documentation plus stickers and labels, it seems to me that any future FCC inspection will take the matter of a professional ground safety install as being non-negotiable, since it involves what I think Marshall termed "NEC vs FCC." And, all things being political, it leaves the part15er looking pretty sharp.

Later I am back to AMEND this blog:

An off-stage voice reminded me that a 3-meter antenna right AT ground level can meet safety AND radial needs, but come to think of it this blog is about antennas raised on poles, so the Amended remark is slightly off subject, but a genuine fact, in any case.

It all helps. Someday when I can afford a Rangemaster, I'll take a shot at developing a good ground radial system for it for use on land.