Low VS High Angle Radiating Antennas

[kb5udf]

Hi All,

I have often heard it said/written that at 144/440 mhz frequencies a 1/4 wave
vertical on a mobile is a 'high angle radiator.' Conversely, I have heard it said
that at HF/Shortwave frequencies, a 1/4 wave vertical or mobile screwdriver whip
is a low angle radiator, better at DX than for NVIS/shortrange communcations.

My question is this, why (if at all) do lower frequency hf transmission behave
differently on a 1/4 wave vertical than on VHF/UHF frequencies?

 

[nd5y]

Normally VHF/UHF antennas are many wavelengths above ground and aren't affected by ground reflections as much as an HF antenna. A horizontal antenna at HF which is around 1/4 wave or less above ground will have a very high radiation angle because of ground reflections. A 1/4 wave VHF/UHF antenna has a higher radiation angle than a 6dB or 9dB collinear or dipole array but it has a lower radiation angle than a HF dipole or wire antenna close to the ground.

 

[LarrySC]

Hams use 5/8 wave mobile ant's to operate thru FM repeaters. The lobe is elevated higher on 5/8 and is more suited for high elevation repeaters. Should have no effect on receiving or scanning.

 

[MB]

Hams use 5/8 wave mobile ant's to operate thru FM repeaters. The lobe is elevated higher on 5/8 and is more suited for high elevation repeaters. Should have no effect on receiving or scanning.

Not true..

The radiation pattern is a "reception pattern" as well, since it also describes the receiving properties of the antenna.

A 1/4 wave antenna (0dBd gain) radiates and receives more energy higher in the vertical plane to reach and receive radio communication sites located in higher places (tall buildings, mountains). Useful in mountainous and metropolitan areas with tall buildings.

A 5/8 wave antenna (3dBd) radiates and receives the maximum amount of signal at the horizon while maintaining a good high-angle signal. This is good for people who live in suburban or rural areas.

 

[n8chb]

Hi All,

I have often heard it said/written that at 144/440 mhz frequencies a 1/4 wave
vertical on a mobile is a 'high angle radiator.' Conversely, I have heard it said
that at HF/Shortwave frequencies, a 1/4 wave vertical or mobile screwdriver whip
is a low angle radiator, better at DX than for NVIS/shortrange communcations.

My question is this, why (if at all) do lower frequency hf transmission behave
differently on a 1/4 wave vertical than on VHF/UHF frequencies?

For many years I operated mobile on both vhf and hf.
First of all it's almost impossible to use a 1/4 wave antenna because it would have to be several feet long. (one Exception 10 meters) so most are loaded coil type and they often have very complex patterns that are hard to compare to a fixed 1/4 wave station antenna.

My personal experiance using vhf antennas is I had the best luck with a plain old 1/4 wave
Motorola type job mounted on the truck deck and roof also.
I ran extensive tests and found due to the way loaded antennas bend at highway speeds
the 1/4 wave antennas always worked 2 to 3 times farther than anything else and had much less fade going up and down hills etc.

That's my take from personal experiance.

Roger Breckon - n8chb

 

[MB]

My personal experiance using vhf antennas is I had the best luck with a plain old 1/4 wave
Motorola type job mounted on the truck deck and roof also.
I ran extensive tests and found due to the way loaded antennas bend at highway speeds
the 1/4 wave antennas always worked 2 to 3 times farther than anything else and had much less fade going up and down hills etc.

That's my take from personal experiance.

Roger Breckon - n8chb

I am also noticing that.

I have an 5/8 wave 3dBd VHF-hi antenna on a bracket mount on my front fender / hood and I have a 1/4 wave mag mount on my roof. The 1/4 seems to pull the signals in a lot better. I will pick up things on the 1/4 wave while the 5/8 wave will be deaf. This also might have something to do with the mountains I live in.

[of course this isn't an accurate test because of mutiple factors - different locations, different mounting systems, different radios, ect... - this is just a generalization}

Anyone else experience the same?

 

[Al42]

Hams use 5/8 wave mobile ant's to operate thru FM repeaters.

Nonsense, Larry. We use 1/4 wave antennas, 5/8 wave antennas, colinear array antennas - just about anything that'll fit on the car without upsetting the law/wife (as applicable). I've seen short towers (with rotators) mounted on cars too.

 

[Al42]

I have an 5/8 wave 3dBd VHF-hi antenna on a bracket mount on my front fender / hood and I have a 1/4 wave mag mount on my roof. The 1/4 seems to pull the signals in a lot better. I will pick up things on the 1/4 wave while the 5/8 wave will be deaf.

The "distance" between the hood (or trunk) and the roof of most cars ia about 5db, so a 3db gain added to a 5db loss gives the 1/4 wave roof mount about 2db advantage. And, if the repeaters you're trying to hit are elevated more than a little, the 1/4 wave has another few db advantage. (And any signals off the back of the car can be as much as 20db down to the hood mounted antenna - it depends on the sizes of the components of the passenger compartment.)

 

[n8chb]

I am also noticing that.

I have an 5/8 wave 3dBd VHF-hi antenna on a bracket mount on my front fender / hood and I have a 1/4 wave mag mount on my roof. The 1/4 seems to pull the signals in a lot better. I will pick up things on the 1/4 wave while the 5/8 wave will be deaf. This also might have something to do with the mountains I live in.

[of course this isn't an accurate test because of mutiple factors - different locations, different mounting systems, different radios, ect... - this is just a generalization}

Anyone else experience the same?

The tests were compared against the way you mounted the antennas.

My conclusion was simplex operation and not working repeaters.
Also my antennas were mounted by drilling holes into the body of the vehicle.
I traded in a car every two years or so and never had a problem selling them by leaving the mounts on without the antenna.

The position and way an antenna is mounted greatly effects the pattern it will produce.
Drilling through the body of the vehicle and using a 1/4 wave makes better use of the mass of the car usually that area will be the engine block.
"the greatest amount of energy will be in the direction of the greatest amount of mass"

One explaination could be with a kids ballon (color is optional). Thinking of the ballon as rf energy the shape and direction of force will be determined by the location of the greatest amount of mass. Unless you want to work the space station the ideal pattern would be if you pushed downward from the top of the envelope.

Antenna wave propagation does not have to be rocket science if one were to follow
some basic things and I'm sure others can offer differant explainations but that's the best I can do for now.

73,

Roger - n8chb

 

[N_Jay]

Where do people get this stuff? :roll:

Roof to trunk deck 5 dB? maybe in some directions, but not in all.
I have never seen a rule of thumb, but 5dB is high from experience.

Radiation in the direction of the "Mass"???:roll:
How about in the direction of the best ground plane (sheet metal) , the engine is irrelevant.

 

[n8chb]

Where do people get this stuff?

I get my "stuff" or experiance from doing it and not from reading some study made by some who claim to be experts or have an investment in selling antenna products.

I might have trouble explaining what I mean and I welcome it to be picked apart but I think for the most part all commercial mobile antenna installations are similar for a good reason.

As strange as it might sound a greater amount of folks put more effert into the radio equipment and very little if any into the antenna system.

That's what I thunk,

Roger - n8chb

 

[kb2vxa]

Hi all,

Re: Al42;
"I've seen short towers (with rotators) mounted on cars too."

Something like THIS?

Now to get serious and keep it simple, a quarter wave vertical regardless of the band it's used on has a high vertical radiation angle, period. Loaded HF verticals be they mobile or out in the back yard are very unpredictable while full length radiators are. Longer radiators (up to a point) have lower radiation angles which is why Amateurs use 5/8 wave for the most part for 2M mobile and colinears on 70cM. In mountainous areas 1/4 wave works better because it's usually UP to the local repeaters. They often have the antennas mounted upside-down to fill in the shadow close in to the tower. BTW, if you may wonder why WABC 770 in NY has such a strong signal it's because they're one of the few that use 1/2 wave towers.

Never mind the usual rebuttal from the usual quarter, the rest including myself are busy laughing at the picture.