Final Question: Is the 'ground plane' always same as 'electrical ground'?

[amanisdude]

I know this probably sounds like a VERY dumb question, especially to a seasoned expert, but is the ground plane of an antenna the same as electrical ground? For example, if I were to build a monopole antenna (such as a whip antenna), would the radiator mast have to be perpendicular to an electrical ground plane, or does the "ground plane" have another meaning in radio communications jargon?

I could further augment this question with the dipole antenna. Is one of the "poles" in a dipole antenna electrical ground? (That is, is one of the rails in a feed line, such as a ladder line or a coaxial cable, connected directly to electrical ground, such as earth ground or the negative terminal on a battery?). Or does, as I currently understand it, one of the poles receive and transmit an inverted, out of phase ("mirror") signal.

With regard to a monopole antenna, I assume that "ground" is "conventional ground" as used in electronics, since the whip antenna of a car usually uses the car body as a ground plane, which is electrical ground, but please clarify if I am wrong.

I'm sorry to post this noobish question to a ham radio forum, but I am surprisingly having a VERY difficult time getting this question answered. If this question is too dumb to answer, could someone please point me to a good resource that would confirm or correct my hunch? Thank you so much.

Also, if this question (along with my other questions on this board) are not appropriate for this forum, please let me know so I know to stop posting here. Thanks.

-amanisdude
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[kc5fm]

I know this probably sounds like a VERY dumb question, especially to a seasoned expert, but is the ground plane of an antenna the same as electrical ground? For example, if I were to build a monopole antenna (such as a whip antenna), would the radiator mast have to be perpendicular to an electrical ground plane, or does the "ground plane" have another meaning in radio communications jargon?

I think you may be confusing counterpoise and electrical ground. Perhaps, reading this will help.

Using your dipole and ground plane examples, the dipole has a counterpoise. The counterpoise for the ground plane is the radial system.

That does not mean one should not use an electrical grounding system separate from the antenna. Having your equipment grounded may protect it from lightning, though the best protections from unplugging antennas and power lines during lightning events.

Hope that helps.

73

[K7KBN]

A standard dipole doesn't require radials or a ground plane because "both halves" of the antenna are right there, separated by the center insulator. You often will see dipoles fed directly with coax cable, and this results in one half of the dipole connected to the chassis of the radio by means of the coax shield. This is not necessarily bad; I've built hundreds of dipoles this way and they've all worked.

A quarter-wave vertical, on the other hand, represents only one-half of the antenna. The center conductor of unbalanced feedline (e.g. coaxial cable) is typically connected to the vertical portion, and a number of quarter-wave radials are connected to the shield. These radials form "the other half" of the antenna. The radials don't have to be connected to any other "ground"; they are only there to provide a complete circuit for the antenna current which is RF.

The "whip" antenna isn't a "monopole" - it's a whip, cut for a particular frequency. And typically it uses the metallic structure of the vehicle as the ground so the RF has a complete circuit in which to operate.

The RF ground plane is not really the same as the electrical ground connected to the negative battery terminal, but it usually serves both purposes. You don't need to worry about voltage and current fields at this point in your journey.

[amanisdude]

Thanks so much, K7KBN! That cleared things up nicely.

So, if I'm understanding this correctly, for both dipole and monopole antennas, the outer shield of a coaxial feed line is connected to the chassis, which is usually electrical ground (on a good build, that is), but does not need to be. Does this mean that the outer shield of the feed line can be connected to the chassis at both source and termination? Also, is the same true for a ladder line? That is, can one rail of the ladder line be connected to the chassis/ground at the source and/or termination, or does a ladder line need some sort of RF connection for each rail?

One final question: between a quarter-wave or a half-wave whip antenna, which is generally more efficient?

Sorry for this barrage of questions. I am trying to extend the range of a simple car FM transmitter (to its legal limits, of course ), and I'm becoming fascinated with antennas in the process. Thanks for helping me understand this!

-amanisdude
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[5B4AJB]

which is generally more efficient?

Generally speaking, the bigger one

[K7KBN]

Thanks so much, K7KBN! That cleared things up nicely.

So, if I'm understanding this correctly, for both dipole and monopole antennas, the outer shield of a coaxial feed line is connected to the chassis, which is usually electrical ground (on a good build, that is), but does not need to be. Does this mean that the outer shield of the feed line can be connected to the chassis at both source and termination? Also, is the same true for a ladder line? That is, can one rail of the ladder line be connected to the chassis/ground at the source and/or termination, or does a ladder line need some sort of RF connection for each rail?

One final question: between a quarter-wave or a half-wave whip antenna, which is generally more efficient?

Sorry for this barrage of questions. I am trying to extend the range of a simple car FM transmitter (to its legal limits, of course ), and I'm becoming fascinated with antennas in the process. Thanks for helping me understand this!

-amanisdude
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At VHF, dipoles will tend to be a lot more bidirectional than in the HF spectrum at the same height. If this is what you want, then the dipole is probably better. The 1/4 wavelength vertical/whip (NOT a monopole), on the other hand, will transmit 360° quite efficiently if it's provided with several radials of the proper length. The quarter-wave whip is also generally a very good match to 50 ohm coax. Now, half-wavelength verticals will not match coax feed without some sort of matching at the feed point.

The coax shield is normally connected directly to the transmitter chassis by means of the connector (PL-259 or whatever). In a dipole, the center conductor goes to one side and the shield to the other. If you're using a balun at the feedpoint, connect the coax to whatever receptacle the balun manufacturer provides.

Ladder line/window line/open line/parallel feeders/balanced line (different names for basically the same thing) connect to the transmitter or antenna matcher through a balun, thus keeping the length of coax as short as possible to reduce losses. Short piece of coax from the transmitter to the balun, and balanced line from there to the feedpoint.

[amanisdude]

Thanks guys! You're helping me a lot! Just a couple more things to clear up.

Generally speaking, the bigger one

Does this mean that a 1/2-wavelength antenna would be better than a 1/4-wavelength without amplification, or is bigger only better when the output signal from the transmitter is amplified? Also, does the "bigger one is better" philosophy apply to antennas larger than 1/2-wavelength? (For example, would a 5-wavelength antenna work better than a 1/2 wavelength?)

The 1/4 wavelength vertical/whip (NOT a monopole), on the other hand, will transmit 360° quite efficiently if it's provided with several radials of the proper length.

I assume all the radials are also 1/4 wavelength in such a setup.

...half-wavelength verticals will not match coax feed without some sort of matching at the feed point.

Now, just for learning's sake, how would one match the verticals to the coax feed at the feed poin for a half-wavelength antenna?

Ladder line/window line/open line/parallel feeders/balanced line (different names for basically the same thing) connect to the transmitter or antenna matcher through a balun, thus keeping the length of coax as short as possible to reduce losses.

Does this mean that, if building your own rig, you can simply eliminate the coax component altogether and solder each respective cable in the ladder line directly to the transmitter and chassis, thereby eliminating the need for a balun?


Thanks for your continued explanations! I think I'm finally beginning to understand antennas well enough to move to implementation.


-amanisdude
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EDIT: Made some clarifications to a few of my earlier questions/statements. Also, I'm sorry for this apparently never-ending newbish question cycle. I am building a low-power amateur rig with no prior experience in radio design, and I just need to be clear on a few things. Thanks so much for your patience.

[K7KBN]

Radial length depends on whether the radials are very close to (or buried in) the soil, or if they're elevated. If the antenna is on top of your house, for example, you couldn't get the radials down to the earth, so they'd have to be elevated. Elevated radials do have to be 1/4λ long. Non-elevated radials should be as long as reasonably possible.

For matching a 1/2λ vertical, you'd have to measure or calculate the reactive portion of the feedpoint impedance and install the proper components to provide equal reactance in the opposite direction, plus figure a way to get the resistive portion down.

Grab a copy of the ARRL Antenna Book. More information there than I can provide over a forum.

[amanisdude]

Thanks K7KBN!

I think I will grab a copy. I have far too many questions to ask on this forum anyway. Thanks for being patient with me!


-amanisdude
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[martyspaulding]

Let me follow this all up with another newbie question.

I have a 32' tower. At the top is a balun with 80m and 40m wire antenna arrays ... which are tied toward ground at less than 180 degree directional angles. (They're tied to off-ground points as available which are not 180 degrees apart, and the two different wire runs are tied at different points/angles as well).

Question 1: Does the differing angles of the two wire runs cause any MAJOR issues/conflict ?
Question 2: Does the lack of exacting 180 degree runs cause any major issues (I assume it will affect directionality more than anything else) But is there going to be any loss or conflict otherwise ?
Question 3: I have a standard NEC ground rod and the tower is strapped to that. Would I gain any benefit from running ground radials and tying them to the tower ? How many and what length ?

[K7KBN]

"Wire antenna arrays"? I have to assume this means you have a pair of independent dipoles, one for 80 and the other for 40. I'm puzzled also by the "less than 180 degree directional angles" statement. Are the dipoles hung as inverted vees?

A traditional dipole is pretty much horizontal and straight for its full length, and the feedline runs away at 90 degrees from the feedpoint. An inverted vee has the center higher than the ends, with the vertex angle no smaller than 120 degrees to avoid too much phase cancellation.

Your antenna, with the center at only 32 feet, isn't going to have much directivity on either band. Dipoles don't need radials, either. That ground rod your tower is bonded to MUST also be bonded to your electrical service entry ground with at least 6AWG wire. (NEC requirement).

Experiment with antennas. A lot of antennas that "shouldn't" work might do just fine in the right circumstances.