ARRL Extra Class Q&A #1 - Radiation Resistance

[Tom WA3KLR]

As a result of my studying for the Extra Class written test, here is a question & answer from the ARRL Extra class 2002 question pool that irks me.  (Two or 3 more topics to be posted on some other Q&A's also.)  I am interested in your expert comments.

“E9A07  What factors determine the radiation resistance of an antenna?

A.   Transmission line length and antenna height.
B.   Antenna location with respect to nearby objects and the conductors’ length/diameter ratio.
C.   It is a physical constant and is the same for all antennas.
D.   Sunspot activity and time of day.

Answer = B.  An antenna’s location with respect to nearby objects – especially the earth – helps determine the radiation resistance.  So does the conductors’ length/diameter ratio.”

B is the ARRL’s correct answer.  I disagree with it though.  It appears that the question is considering a vertical antenna system specifically (consisting of a vertical element and a ground plane).  The radiation resistance of the vertical element is based on the height of the element and it’s diameter.  The fact that the ARRL includes nearby objects as a factor in the radiation resistance is what bothers me.  I would think that nearby objects cause an additional loss, but this should add an additional series resistance to the feed point, in series with the radiation resistance, giving an increased total feed point resistance.  In other words, yes, the antenna radiation efficiency will lower with nearby objects, but the radiation resistance itself remains unchanged.

Finite conductivity of the real ground is a loss which will add a series “ground loss” resistor to the feedpoint resistor model string.

From ARRL Q&A E9A14 – antenna efficiency = (radiation resistance/total resistance) X 100 %.

Perhaps the nearby objects can be looked as a “widening” of the vertical radiator.  In this case the radiation resistance could lower, but again this appears to be a very lossy inadvertent vertical radiator too, adding loss resistance to the total resistance model string.

If you could do an empirical experiment (HI) and measure the antenna feedpoint impedance and then take away the nearby objects, you would have your answer as to what the net effect is.   The lowering of the radiation resistance should be small if at all, compared to other losses added.

Any antenna gurus out there with comments on the correctness of Answer B.?

[Bill, KD0HG]

I agree 100% with you Tom.

Radiation resistance is NOT the same thing as feedpoint impedance, something the question's writers have all wrong.
For that cited vertical Marconi antenna, radiation resistance is a sole function of an antenna's physical length in degrees.

You can fold a vertical, you can tap the feedpoint at places other than the base, you can change conductor material, you can add nearby metallic objects, all you're doing is changing the feedpoint Z, I2R losses, or altering its *electrical* length.

An infinitely-thin resonant 1/2 wave dipole has a radiation resistance of ~75 ohms, a resonant folded dipole might have a feedpoint Z of 300 ohms, but its radiation resistance is still 75 ohms, *not* 300 ohms. You can play with conductor diameters or make a three-wire folded dipole with impedances all over the place, but the radiation resistance is still ~75 ohms.

Answer B really isn't accurate, but it's more correct than the others.

Good catch.

[Steve - WB3HUZ]

Agreed. But your point is true no matter the antenna configuration (horizontal or vertical), as Bill pointed out with his dipole examples. The question would have been better worded if the term feedpoint impedance was used, since the ground and other nearby objects not only affect the resistive part of the the feedpoint impedance, but the reactive portion too.

[AB2EZ]

Tom

As I think you will agree... 99.9% of all of the people who take ANY engineering license examination (including Amateur Radio License examinations and Professional Engineer license examinations) will, at best, memorize the answers to these types of questions (if they are taken from a question pool)... or guess the answer.

When I took the tests (general - extra class) in 1998 to obtain my license, the VE's were amazed that I actually was calculating the answers to the technical questions... rather than drawing on memorized answers.

It is hard to imagine why anyone would be required to actually understand these types of questions... and to actually understand the answers to these types of questions... just to obtain a license to operate an amateur radio station. The vast majority of amateur radio operators will never need to know these things... and will never really know these things.

It would, in my opinion, be far better if the licensing process focused on safe operating practices (operator safety and the safety of others), and knowledge of the rules and regulations... just like a driver's license exam.

It is also clear, as you point out, that even the people who make up these questions often don't understand the question they are asking... and often don't understand what the correct answer is (assuming the question is well enough posed to even have a correct answer).

When I helped students prepare for taking the Professional Engineer license exam... I was amazed at what a farce it was. The questions and answers from prior exams (used by the students to prepare for their exams) demonstrated that the folks who made up the questions, in many cases, didn't even understand the questions they were asking... and certainly didn't understand what practicing engineers needed to know to do their jobs professionally.

Best regards
Stu

[KB2WIG]

Stu sez   "When I took the tests (general - extra class) in 1998 to obtain my license, the VE's were amazed that I actually was calculating the answers to the technical questions... rather than drawing on memorized answers."

When I took my first test in 95,  Ed, one of the administers, had a tie clip ( yes, he wore a tye ). After the code test,  and the No voice , and Teeck test, ( and I took the gen test, and failed that by 1 question - didn't study for it). The tie guy congraduated me and I commented  on his tie clip - said it looked like the tube in the HP vtvm's.  He was suprised that someone new something about electronics.... Turned out we actually had a few mutual friends....     

Keep up the good fight -  try to have Some understanding of the actual technoloogy... Maybee this can be the reason for an Extra Class license.  The tests can have some non memorizable parts with calculations. The RLC QueSTions's on the Extra test can be talored so the numbers/ answers change.... 

My tirade....  A Teeck class that I know can't do fractions --   No idea of what Ohms law is --  and hes got a technical license??

[Rob K2CU]

HI Tom,

Totally agree with the BS in the wording of several questions in the FCC exams. I found the same BS questions when I got my First Class (before they dumbed down that).  A lot has to do with the meaning of words and what is implied. It becomes subjective in many instances. When I first read this type of question when I studied for my Extra several years ago, the first thing that came to mind was texts by Ramo, Plonsey, and Jasick.  I remembered that radiation resistance was an effect produced by the creation of the EM fields around the antenna and reflected back into the structure as it attempted to match the 377 Ohms of free space to the transmission line.  In many derivations, they work backwards and ask "What must have been the relation between applied voltage and current to produce the resulting emission."

In any case, with these type questions, you find that there seems to be a deliberate attempt to make the meaning somewhat abiquous, if not right incorrect. You find yourself realizing that you just have to understand what they are asking and expect for the answer. In a way, it is a good question and good to share with the forum because it makes us think.  When you find these questions, especially the ones that incorrectly put together related terms in a manner that  causes confusion, such as confusing energy with power, don't over analyze the thing. Just realize the trick question,  memorize their answer, and then do your best to forget it and remember the correct technical answer so you won't be confused about it down the road.

[WA1GFZ]

When I took my extra test at the Customs House in Boston. The fcc guy was ripping into a jn for trashing his neighbor's TV. I think his name was j. Sarno and never wanted to tick him off. Back when we were very afraid.....

[k4kyv]

When I took my extra test at the Customs House in Boston. The fcc guy was ripping into a jn for trashing his neighbor's TV. I think his name was j. Sarno and never wanted to tick him off. Back when we were very afraid.....

Years ago during the great AM vs SSB wars of the mid to late 60's, Con, W4EBG in Paducah, KY was notorious for the language he would use on 75m AM after a few 807's (language that to-day wouldn't even raise an eyebrow at the post-Janet Jackson FCC.)  He never had his licence suspended or revoked, but when it expired in the late 60's, the FCC refused to renew it, citing the "character" issue.  He probably could have taken legal action and got his renewal, but he decided to just let it drop, even though several lawyers who were also licensed hams offered him legal services free of charge. He died a few years later without ever returning to the air.

Sometime during the period when this was still an active proceeding at the FCC, amateur exams were still administered by FCC personnel, and incentive licensing had just been implemented, WA4JEW (also now SK) who often found himself in QSO's involving Con, went down to the quarterly FCC examination point to upgrade from General to Advanced.  He filled in his application form and handed it to the FCC examiner.  The examiner took a look at it, smiled, and announced to the whole room full of licence candidates: "Well look here. WA4JEW. This is one of old Con's buddies."

[Bill, KD0HG]

LOL, Don, here's a true story.

In the early-mid 1970s , I used to raise hell on 75M SSB with some friends down around 3825 KC. One of them was WA9*ZF, Wayne from Indiana, who joked on the air about having a "big" amplifier Heh.(he didn't) and the FCC did inspect him once. They walked into his house demanding to see his logs and linear. He pointed at a little PW pair of 4CX250Bs, told them that was it, and they kept demanding to see his non-existing 4CX5000 rig.

Not long afterwards,, I called the Chicago FCC field office with a question about broadcast regulations. I introduced myself by name and the conversation went like this:

FCC inspector: "Hey, are you a ham operator?"
Me: Yes.
FCC inspector: "What's your call?"
Me: "WA9OZC...Why?"
FCC: "Did we ever bust you or inspect your station?"
Me: "Why...no"
FCC: "Well, we were going to, AND WE SHOULD HAVE!..Now, what do YOU want and who did you say you were working for?"
Me: "Ahhh, nevermind!"
(Click)

True story, Don.

[W2VW]

An infinitely-thin resonant 1/2 wave dipole has a radiation resistance of ~75 ohms, a resonant folded dipole might have a feedpoint Z of 300 ohms, but its radiation resistance is still 75 ohms, *not* 300 ohms. You can play with conductor diameters or make a three-wire folded dipole with impedances all over the place, but the radiation resistance is still ~75 ohms.

Answer B really isn't accurate, but it's more correct than the others.

Good catch.

If the folded dipole exibits a feedpoint Z of 300 ohms and is resonant then the radiation resistance in practice will be very close to the 300 ohm impedance. The impedance is resistive and is 300 ohms.

[WA1GFZ]

Sarno had a window behind his desk and we were up in the air pretty far. I was just thinking about the poor slob jn going through the glass. He really went crazy when the guy told him that he moved his antenna away from the neighbor's house thinking it would eliminate the tvi.
Then I went back to my test.

[K1JJ]

In the early-mid 1970s , I used to raise hell on 75M SSB with some friends down around 3825 KC. One of them was WA9*ZF, Wayne from Indiana, who joked on the air about having a "big" amplifier Heh

Bill,

There was a similar guy on at that time (75M ssb, early 70's) with a call like WB9---  from Chicago. He claimed to have a dipole at 100' and running 5KW. He was always stirring up the band, much like the WA4Dawg used to do -  had a good voice and speaking manner like a DJ.

I've tried to remember his call and always wondered what ever happened to him. Does this ring a bell?

T

[WA1GFZ]

wa4dog 40 grit on two wheels

[Joe Long]

Prior to his employment at the FCC Jerry Sarno was a vice detective with the Boston P.D. He replaced Nathan Hallenstein as top dog at 1600 Customhouse in Boston {FCC office}. Hallenstein was a ham. Sarno was not. Local hams had little respect for Sarno. Nathan was a great guy.  Joe

[WA1GFZ]

So that's why I thought he acted like a jerk.
I remember handing in my test and him giving me a look like " I know you cheated"
I wonder who got him the job

[Bill, KD0HG]

Bill,

There was a similar guy on at that time (75M ssb, early 70's) with a call like WB9---  from Chicago. He claimed to have a dipole at 100' and running 5KW. He was always stirring up the band, much like the WA4Dawg used to do -  had a good voice and speaking manner like a DJ.



I've tried to remember his call and always wondered what ever happened to him. Does this ring a bell?

T

Yes, there was a group of Chicago and Midwest-area miscreants on then. Absolute crazies.  Maybe you're thinking of WA9PMQ. They called themselves "NASAC", I can't spell out what that stood for here...

IIRC a couple of that group actually were running some big forced air cooled ceramic strap. There was one guy from that group that was running an RCA 6166, a 12KW plate dissipation VHF TV transmitting tube. I'd love to pass along his name and call but that wouldn't be sporting...He ended up working for the FCC after graduating.

!