You read a lot about "they need to fix the ground system" when an AM station is perceived to have coverage issues. Has anyone seen a substantial improvement (by substantial, I mean the signal went from "not being listenable" to "being listenable") after ground system upgrades/repairs/replacement? I've seen two ground systems replaced...one was just ancient & literally falling apart in the ground (non-d) & the result was a more stable base current reading, but zero improvement in field strength. The other was a 2 tower DA where a farmer plowed up almost the entire ground system--fortunately the feed lines & sampling lines were buried deeper than the radials ("I found these copper wires attached to my plow"--that GM didn't last long after leasing the antenna field for farming)...it caused changes in the otherwise excellent stability of the DA, but once the DA numbers were dialed back in, no significant change in field strength before or after the ground system was replaced. These are the only two cases I've seen in my 42 years of mostly FM based engineering. Is my limited experience typical or does the ground system actually play a 'make it or break it' difference in how well an AM antenna radiates? Forgetting about the theory, I'd enjoy hearing about real world results related to AM ground system modifications.
-
Get involved.
We want your input!
Apply for Membership and join the conversations about everything related to broadcasting.
After we receive your registration, a moderator will review it. After your registration is approved, you will be permitted to post.
If you use a disposable or false email address, your registration will be rejected.
After your membership is approved, please take a minute to tell us a little bit about yourself.
https://www.radiodiscussions.com/forums/introduce-yourself.1088/
Thanks in advance and have fun!
RadioDiscussions Administrators
Ground System Importance
- Thread starter BobOnTheJob
- Start date
This was initially studied in great depth by George Brown, Phd and others at RCA back in the 1920’ sand 30’s. Later on, Jack De Witt of WSM fame also confirmed those findings by extensive field measurements and test. All of that is published in the IRE and IEEE journals in those years. Essentially what it says is you reach about 95% of the expected field with as few as *6* radials and almost 100% with *8*. *120* 90 degree radials is what the perfect model was and provided the “perfect” symmetrical counterpoise. In later years, there was discovery that the depth of the wires in the ground had some impact on near field signal strength but was less evident in the far field (beyond about three wave-lengths). This was conducted by Palmer Greer and Associates. More recently, Kintronics and DLR conducted and tabulated some extensive field measurements on various AM antenna configurations including series and shunt (uni-pole) feed systems. That test served to confirm what the others discovered years back but mainly debunked all of the wild efficiency and bandwidth claims made by the uni-pole “snake oil” salesmen. It’s on their website as white paper.
Outside that, my findings are pretty much what yours are with the possible exception of a few multi-element arrays that exhibit very low driving point resistances and even slight disturbance in the ground currents can disturb the operating parameters. I know several who replaced the entire system with disappointing results. The results are usually spun as positive because of the amout of money invested and no one will admit publically it was all for no gain. One exception is a Class-4 that we discovered the ground system was properly installed but had never been connected to anything. First clue was the drive Z was real high for a 75 degree tower.
Outside that, my findings are pretty much what yours are with the possible exception of a few multi-element arrays that exhibit very low driving point resistances and even slight disturbance in the ground currents can disturb the operating parameters. I know several who replaced the entire system with disappointing results. The results are usually spun as positive because of the amout of money invested and no one will admit publically it was all for no gain. One exception is a Class-4 that we discovered the ground system was properly installed but had never been connected to anything. First clue was the drive Z was real high for a 75 degree tower.
Well, we LMA'd a kilowatt daytimer with no ground system. Previous owners had sold off the land around the tower until the only land owned by the station was a 100 X 25 foot lot the tower (Blaw-Knox 225' self supporter) stood on. We got in just after an FCC inspection, inspector claimed transmitter was running over-power, based on base impedance shown in the license.
Since that would have required a BC-1T to run at 1200 watts, we kind of concluded the base impedance had shifted. The whole site was "hot." We built a box around the tower base with railroad ties, ran a copper strap along the top, then saturated the area around the base of the tower with a variety of different color #10 and #12 copper we got cheap (tag ends) from an electrical distributor--all tied together under the base of the tower and to the strap, which was then tied into the ATU and transmitter. Not great, but made an immediate difference.
Then we sneaked some long radials into the adjacent lots on two sides of the tower, which were just an open field at the time. We then re-measured the base impedance and found it was much closer to the original licensed parameters.
Would a full ground system made that much more improvement? Hard to say, in this case the fat self-supported saved the day. Incidentally, the station is still there (different owner), but the adjacent field is now a funeral home and parking lot.
Since that would have required a BC-1T to run at 1200 watts, we kind of concluded the base impedance had shifted. The whole site was "hot." We built a box around the tower base with railroad ties, ran a copper strap along the top, then saturated the area around the base of the tower with a variety of different color #10 and #12 copper we got cheap (tag ends) from an electrical distributor--all tied together under the base of the tower and to the strap, which was then tied into the ATU and transmitter. Not great, but made an immediate difference.
Then we sneaked some long radials into the adjacent lots on two sides of the tower, which were just an open field at the time. We then re-measured the base impedance and found it was much closer to the original licensed parameters.
Would a full ground system made that much more improvement? Hard to say, in this case the fat self-supported saved the day. Incidentally, the station is still there (different owner), but the adjacent field is now a funeral home and parking lot.
So would that station be on a grave yard frequency? Lol! I couldn't resist.
Watt Hairston said:
And which real-world study showed that even for the conductivity of the sandy soil of New Jersey (4 mS/m or less), the groundwave field intensity that BL&E measured at 3/10ths of a mile using 113 0.41-wavelength buried radials with their experimental setup was within several per cent of the theoretical maximum for a PERFECT r-f ground and ground plane, for monopole heights of about 60 to 90 electrical degrees in height. Fewer and shorter radials did not produce the same result.
The electrical height of a monopole determines its radiation resistance, and that is fixed by Physics.
The radiation resistance of a given monopole is independent of the ohmic loss in the r-f ground it is driven against, and other losses present in the "ATU" at the base of the antenna.
The ratio of that radiation resistance to the sum of the other ohmic losses present in the antenna system PLUS that radiation resistance determines the groundwave field intensity per watt of available power that will be produced at a given distance from that antenna system.
//
The pattern was OK but the monitor indications were whacked. We spent a week checking sample lines, feed lines, adding 4" strap to the 4 ATU's that had only one 2" strap on them, and did not get the monitor to go all the way back in. Walking around we noticed a torn up thin 3" strap sticking out of the ground. Found the other end, hooked them together & it was fixed! Found out the need to walk nearly the entire property looking for anything out of the ordinary.
Although my experience is largely theoretical, I have extensively studied the field strengths of many AM stations in areas in the Great Lakes region.
I suspect that there are factors other than conductivity, such as magnetic properties of the soil and actual dielectric constant, that are involved in signal prediction and that are not taken into much consideration by the FCC. It makes liitle sense to put Epsilon=15 on the groundwave graphs and consdier that the Great Lakes have a dielectric constant of 78. I have e mailed Mr. Fry in the past, and I wonder if he has any thoughts on that. Temperature is also an overwhelming variable as evidenced by summer winter variations. It's not just snow and rain either.
Obviously, Longley Rice has made some strides in FM and TV FS prediction, taking some of these factors and more into account. Vegetation, elevation, and wave bending over hills also seem to affect AM BC frequencies.
I suspect that there are factors other than conductivity, such as magnetic properties of the soil and actual dielectric constant, that are involved in signal prediction and that are not taken into much consideration by the FCC. It makes liitle sense to put Epsilon=15 on the groundwave graphs and consdier that the Great Lakes have a dielectric constant of 78. I have e mailed Mr. Fry in the past, and I wonder if he has any thoughts on that. Temperature is also an overwhelming variable as evidenced by summer winter variations. It's not just snow and rain either.
Obviously, Longley Rice has made some strides in FM and TV FS prediction, taking some of these factors and more into account. Vegetation, elevation, and wave bending over hills also seem to affect AM BC frequencies.
BobOnTheJob said:
How does the material presented in this fascinating thread affect the protestations I've seen elsewhere (the poster may have been someone who has also posted in this thread) that a tree growing among the ground radials of a directional AM can seriously affect the measured pattern? I believe that it was the same poster who insisted that vegetation, other than lawn-height grass, growing in the area covered by the ground radials indicates sloppy site maintenance and a high likelihood of out-of-spec performance.
DanStrassberg said:
That parallels a philosophy I developed during my broadcasting years: when I went for an interview there were two places I wanted to see... the restroom and the engineers shop space. The housekeeping of those two areas could tell you a lot about the management mentality of the station..... and the likelihood of out-of-spec performance.
LMAO!
I interviewed for an engineering gig in 1977 at WPFB/Middletown,OH & didn't get it for that very reason...the corporate guy believed that 18" high grass/vegetation would cause his NON-D AM on 910 to suffer from poor radiation efficiency. He asked that question in the interview...I may have answered correctly, but I didn't get the job.DanStrassberg said:
Wonder what some would think of the acres of sagebrush, cheatgrass, and lava rock around one site. Nobody in their right mind would spend the dollars to keep that cut down. However, inside inside and just outside the fences is kept bare and the bathroom at the transmitter is clean.
A FUNCTIONAL ONE????Nostalgia said:
BobOnTheJob said:
I was in transmitter site where there was even a first-aid room, with a bed, oxigen, etc... Ok, it's a 100kw AM site ;D
BobOnTheJob said:
I worked at a transmitter site that not only had a functional bathroom but a phone next to the toilet. The GM wanted to make sure the engineer was available to take a phone call.
once worked for station that transmitter had two bathrooms. One up-stairs, one down. No phone in there though...
That ground system horse has been beaten for 77 years now.
That ground system horse has been beaten for 77 years now.
- Status