The AM Forum

In the 160 meter broadcasting thread K5UJ mentioned this


I would like to understand how tower height for both AM broadcasting, and hear on our bands, the height of a tower or the spacing of towers in a directional array is measured in degrees. I have done a few searches but without a basic understanding I'm no closer to grasping this. If one of you can fill me in hear or point me to an explanation on the web I would appreciate it very much. Remember, I'm looking for both single tower and directional array (multi tower) degrees in height and spacing information. Thanks!

Mike

My guess is that a 90 degree antenna tower is a 1/4 wavelength. One complete sine wave is 360 degrees & is equal to 1 wavelength.

Height (or Spacing) in feet =

   (984 * Electrical Degrees Needed) / (360 * Freq in MHz)

Mike,
When you figure the length of a typical dipole you usually figure it in degrees.  Yup, 1/2 wave dipole is 180 degrees long! It really is that simple.
1/4 wave = 90 degrees
1/2 wave = 180
3/4 wave = 270

and everything in between!
 

You guessed right.

Funny but Ive never done that with a dipole and rarely a vertical but think in degrees all the time when working with phasing lines. Its just the way we we brought up into the hobby Id guess.

Thanks all for the replies. Bud, I think I was expecting the answer to be more complicated. Like understanding Smith charts or the Ubiquitous Conjugate Match.  ;D

Mike

Must be the way we were initially trained, Carl.

Once I got into compliance engineering and started working with the FCC, who list vertical BC radiators in degrees, for instance, I began thinking more in degrees. It just kinda mutated my thinking. But once we realize an antenna doesn't need to be 1/4, 1/2 or 5/8ths wavelength to work well it opens up a whole new world. Particularly for we hams who are forced to work with defined, usually city, property dimensions.

Hi Mike,

I did not think of wave length fractions in terms of degrees until a few years ago, and, like you, had to have it explained to me.  But, it grows on you for it is (to me at least) a more convenient way of expressing a wave lengh fraction ("90 deg." v. "one quarter wave length") but more important, it is more precise in some cases for example, I do not even know what the lowest common denominator fraction is (190/360) for 190 degrees. 

Rob 

When speaking about broadcast vertical radiators, degrees are convenient to convey relative height, driving impedance, current distribution, and expected efficiency - anywhere over the octave-and-a-half band.

Some hams seemed locked into thinking that resonant antennas are superior, when in fact, most broadcast antennas, engineered for high efficiency, are not - even those nominally 90 degrees. 

The most common height is probably 199 feet - to avoid lighting.

Careful experiments done in 1937 by Brown, Lewis & Epstein of RCA showed that monopoles from about 45 degrees to over 90 degrees in height, using 113 x 0.412-wave buried radials produce a groundwave field at 3/10 of a mile that is within several percent of the maximum theoretical field possible using a perfect monopole driven against a perfect ground plane.

This result proves that such systems are radiating about 95% of the applied power.


Licensed AM broadcast stations are required by the FCC to produce a minimum groundwave field depending on their Class.  In many cases this precludes the use of electrically short monopoles.

H.A.R.A.D.

Ham Antenna Resonant Arrested Development.

Otherwise intelligent hams can be found on the web posting how it is impossible to feed a center fed full wave doublet due to the high impedance. They don't seem to understand what happens to the impedance at the shack.

Others throw away the couple extra dB gain from a full wave doublet because they can't come up with coupler adjustments which are repeatable.

Coaxial feed isn't allowed below 28 mhz in my shack.  

Coax, HUH? ? ? ? 

Resonant antennas arent allowed here below 28MHz  ;D  ;D  ;D

All you need to do is figger out how to make it look tasty to the transmitter......

"Degrees" is universally understood in engineering circles and more convenient to remember and store in data than would be odd fractions of a wavelength, whether common or decimal, except perhaps the usual 1/8, 1/4,1/2 and 5/8λ. Also, the figures are easier to plug right into the Smith Chart.

My tower is 127' high.  The FAA guy I talked to 30 years ago looked at his map and told me I could only go up to <150 ft without lighting the tower and submitting paperwork, due to helicopter activity at the near-by army base, even though I do not fall within any of the airport restrictions defined in Part 97.  I suppose the FAA has the final word.  I wasn't interested in going that high with only a single tower anyway. Many structures in this area, some no higher than my tower, are lighted.  I took the FAA guy at his word with nothing in writing, and put up the tower. I checked with the county and they told me I didn't even need a building permit. No-one has ever given me any flack over the tower or not having a light. I figure it must be "grandfathered" in place as-is, by now, even if regulations and permit requirements have changed since then.

I have NEVER fed a dipole with coax.

Mike
I have a couple of books that go into broadcast arrays in depth if you want to look at them. One is old and one is newer. The National Association of Broadcasters Handbook.  The first one is 1960 vintage 5th edition the other is the 7th edition. I found the first one at a hamfest so I decided to buy a new one that was expensive and I like the old one better, they have served me well thru the years. They made me join the association before I could buy a book."Scam"

73 John N8QPC

I have an extra copy of the circa 1960 edition, fair-minus condition.  Anyone have something they wanna swap?

(If so, please reply by PM, not on this thread.)

For some math, here is an example of a station operating at 1.0Mhz.

Lambda  is wavelength, c = is speed of light or of an EM wave is 3x10^m/s in vacuum, Frequency is in Hertz.

1 full wavelength or 1 Lambda = c/Frequency in Hz = 3X10^8/1.0X10^6 = 300 meters = 984 feet which is also a full 360 degrees of wavelength, or 984/360  = 2.733 feet of antenna height/degree.

A 195 degree (0.54 Lambda anti-fade) antenna would be 195 degrees X 2.733 feet/degree = 533 feet.

Since EM waves slow down in conductors about 5%, a rule of thumb is to shorten the antenna by 5% so the cal. would be, 0.95 X 984 ft = 934.8 feet.

An antifade antenna of 0.54 Lambda would then be about 0.54 X 934.8 = 506 feet.

So a quaterwave monopole antenna would be 0.25 (90/360)  X Lambda or,  90 degrees X 2.733 feet/degree X 0.95  = 234 feet.

BTW, 195 degrees/360 degrees = 0.54



Phil - AC0OB

Don what did you think of their book?

John

In many cases the FCC allows more power to meet the field strength requirement.

Carl

The FCC requires a minimum r.m.s. groundwave field intensity at 1 km  based on the class of station, when 1 kW is applied to the antenna system.  It does not permit a station licensed for 1 kW (for example) to install an inefficient antenna system, and use a higher power transmitter to produce the minimum field.

Note that a Class A station cannot produce its minimum field of 362 mV/m at 1 km with 1 kW even when using a 90-degree monopole with 120 x 1/4-wave radials.  It has to install a taller tower.

Most Class A, 50 kW,  24/7 AM stations use 195 degree monopoles.

Paragraph 73.189(b)(2) of the FCC Rules states:

    (2) These minimum actual physical vertical heights of antennas
permitted to be installed are shown by curves A, B, and C of Figure 7 of
Sec. 73.190 as follows:
    (i) Class C stations, and stations in Alaska, Hawaii, Puerto Rico
and the U.S. Virgin Islands on 1230, 1240, 1340, 1400, 1450 and 1490 kHz
that were formerly Class C and were redesignated as Class B pursuant to
Sec. 73.26(b), 45 meters or a minimum effective field strength of 241
mV/m for 1 kW (121 mV/m for 0.25 kW). (This height applies to a Class C
station on a local channel only. Curve A shall apply to any Class C
stations in the 48 conterminous States that are assigned to Regional
channels.)
    (ii) Class A (Alaska), Class B and Class D stations other than those
covered in Sec. 73.189(b)(2)(i), a minimum effective field strength of
282 mV/m for 1 kW.
    (iii) Class A stations, a minimum effective field strength of 362
mV/m for 1 kW.

No need for all the verbiage.

The FCC will allow a power increase on a case by case basis when the signal level cant be maintained for good reason.

I retired over 12 years ago as a broadcast systems engineer for RCA and Harris Broadcast Divisions for about 40 years, so I just refreshed myself on this subject with a broadcast consulting engineer still in practice.  Here is the gist of what he reports.

This only applies only to directional antenna systems.   If the full antenna proof shows that the pattern RMS is below the class minimum the FCC will allow an increase in input power to meet the minimum as long as the underlying theoretical pattern did meet the requirements.  The FCC is fairly picky about it though, and you can't skimp on things like the ground system or radiator heights expecting to make it back with increased common point current.

The FCC will not issue the final license if you did not build the ground system as proposed in the CP (construction permit) application and as specified on the CP, and they won't issue the CP if you propose a deficient ground system.

The ND (non-directional) tower/ground system requirements in 47 CFR 73.189(b) are strictly enforced, and they won't grant increased input power to compensate for poor antenna systems in the ND case.  They will allow nonstandard tower heights and ground systems as long as a full proof is done to back up any claims that the installation meets the minimums fields required for that class.

But for an ND station the FCC will not grant an increase in transmitter power to compensate for a non-conforming antenna system.

(Sorry for the verbiage.)

RF
http://rfry.org

Of course, this is for groundwave coverage, and not very relevant to amateur use, since we depend far more on sky wave, and rarely is groundwave coverage our main concern. In AM broadcasting, skywave coverage is considered "secondary".

The power level is measured right at the base of the tower, so if the matching network and/or feed line have measurable loss, the transmitter itself may run more output power to compensate for that  loss.  The tube (or transistor) generates even more power, since the tank circuit itself is not 100% loss-free. But this loss calculation excludes ground  losses at the antenna, at least in non-directional arrays.

An issue that has been debated regarding amateur power, which I maintain is nitpicking, splitting hairs and much ado about nothing, is whether or not it is "legal" for the transmitter to deliver more power to make up for feed line and tuner losses, if an accurate instrument inserted at the ultimate point of measurement (as close to the radiating element  of the antenna as physically practicable) still indicates legal output power.

However the relative field pattern "launched" from a monopole used by AM broadcast stations is no different than when that monopole is used by amateurs.  In both cases, for monopoles of 90 degrees and less in height the elevation plane relative field is close to the cosine value of the elevation angle, which is unity in the horizontal plane, 0.707 at 45 degrees, and zero at the zenith.

If the r-f ground connection of that system has relatively high loss, then the maximum field intensity it radiates is reduced, but the shape of the elevation pattern is unchanged.  Maximum field launched by that antenna system still is directed in the horizontal plane (contrary to the far-field analyses shown by NEC software).

If the field radiated by a monopole using a poor r-f ground is 3 dB less in the horizontal plane, it is also 3 dB less at all elevation angles above the horizontal plane; angles which produce skywave service so important to amateurs.

So in the final analysis, producing the greatest groundwave signal from a monopole in amateur radio service is as important to amateurs as it is to AM broadcast stations -- even though amateurs depend more on skywave propagation.


In AM broadcast practice, applied power = I²R, where R is the real term of the measured impedance at the tower base, or common point of the antenna system.  That real term includes the resistance of the r-f ground system.

To clarify, I was not disputing that ground resistance is included in the total measured antenna impedance. As you pointed out previously, the FCC sets minimum standards for field intensity, at 1 KW and as I recall it used to be at one mile distance, but I believe the current rules have converted that distance to metric measurement.  They will not allow a broadcaster to skimp on ground system and exclude ground losses in the legal power measurement, for example, taking the field intensity measurements at 2 KW because of a known 3 dB ground loss; I was referring to the FCC's standards as prescribed in the rules, not to physical power and antenna resistance measurement.


In AM broadcasting, the objective is to generate as much ground wave intensity as practicable.  Thus the 195° monopole at Class A stations.  Unless an amateur is interested only in DX, a 195° monopole may not be the best antenna, for example, for coverage of just a few hundred miles.  A shorter monopole, say 60°, might lay a better signal a few states away than would the 195° one, even though it would generate less groundwave intensity locally and less signal strength at far greater distances.

Of course, I am not disputing that once the  length of the shorter antenna has been determined, installing an appropriate ground system and making the adjustments that produce the maximum measured field strength for the local ground wave, will also assure the best performance as a sky wave radiator, even at higher take-off angles more optimum for shorter distances.

But in AM broadcasting, sky wave radiation is almost considered a nuisance.  If someone could invent an antenna that produced only ground wave and no sky wave at all, it would quickly become the industry standard.  "Secondary" sky-wave coverage is given practically no importance to-day, now that practically every square mile of the populated  regions of the continent is saturated with coverage from multiple local stations, hence the degradation of protection the FCC once accorded to distant Class A "clear channels". No broadcaster to-day would choose a 90° tower over a 195° one if he could afford the construction and maintenance of the latter, in order to better serve night-time listeners at the far end of the state or a few states away, beyond the range of reliable ground wave coverage.  An amateur, OTOH, might forego a taller vertical because it would tend to "skip over" the stations he normally talks to every evening.

You better check with the FCC directly and not rely on a CE that is not that familiar with all the rules.

In case of a partially destroyed radial field due to industrial or other enroachment the FCC may OK a power change and the actual amount is their call. They may also require a power reduction at dusk/dark to reduce the skywave back to original levels.

They have also given approval to an elevated radial system which often requires a reduction in power.

Those are special cases.  In the case of accidental damage to the ground system, it's more like a STA to keep the station on the air until the system can be brought back up to specs, but no doubt the FCC would give the station a deadline to get everything back in order.  Regarding elevated radials, those systems were FCC certified for permanent operation and included in the licensing data for the station, and the FCC may impose whatever standards they deem appropriate, at their own whim. In the past the FCC licence certificate that hung on the wall stated the type of antenna, including the degrees of height, power output and the permitted range of base current, the length and number of radials, and details of how the tower is fed.

The FCC has approved at least one low profile vertical monopole that is much shorter than 90°, using a short fat radiator.  I forget its name, but they have recently run ads in several of the broadcast rags. It uses a full radial system, but the radiator is more like four short inverted-Ls arranged back-to-back to each other, and the horizontal sections act like a top hat.  They claim nearly the same efficiency as a conventional 90° vertical. They have grabbed a lot of attention in response to the anti-tower and anti-antenna sentiments that have permeated in recent years.

OTOH, the FCC wouldn't authorise the E-H and Cross-Field antennas, and I haven't seen anything about them lately. I'm not sure if the FCC ever gave anyone the go-ahead to even try one of those antennas experimentally.  Of course, a licensed amateur could have built one for 160m and reported the results, but I never heard of anyone doing that.

Im talking about as replacements for buried radials, not new construction. A new certification was required and as I said there were often power reductions required.

Could you please post the specifics (call letters, links to FCC documentation etc ) supporting your statement?

The FCC requires minimum r.m.s.field intensities at 1 km for the classes of AM stations, but I'm not aware of any maximum field intensity for a given licensed power.

A class C "graveyard" AM station (1 kW, non-directional, 24/7) can produce its minimum field using a monopole of less than 90 degrees and an r-f ground of 120 x 1/4-wave buried radials, but is not prohibited from installing a 1/2-wave or 5/8-wave monopole that, for the same applied power could produce a field at 1 km that is over 1.5X  the minimum value required by the FCC.

Decades ago I was the CE of such a station (1 kW non-D on 1400 kHz), which used a 1/2-wave monopole.