HAMsters: The Case for 1/4 Wave Vertical Antennas for Off-Road Comms. (1 Viewer)

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tl;dr - For 2m HAM radio mobile off-road driving, buy a quality 1/4 wave whip antenna from Diamond or Pulse/Larsen, and call it a day.

To setup the construct, the problem at hand is that I've experienced degraded performance of 2m (145-147mhz) HAM radio reception and transmission over the past several overland expeditions covering hundreds to thousands of miles of driving along straight and lazy interstates to box canyons with severe incline / decline on dirt roads, and everything in between. In practice, the most common antenna type across the traveling group is the 1/4 wave vertical, but my initial choice for greater range and reputation for good performance was the 5/8 wave vertical whip. This article deals specifically with unbalanced, end-fed 1/4w, 1/2w and 5/8w isotropic whip antenna types lacking a perfect ground plane - i.e., the practical application of vehicle-mounted communications equipment for on- and off-road use.

I can almost hear people clicking the 'back' button right now...

Okay - now that I have the attention of the hard-core technical readers (*or the seriously bored-with-nothing-better-to-do crowd)! I'll break this article into a few flowing sections to make it a little easier to read, as the research I've conducted (pun) covers far and wide to pull together concepts relevant to the solutions.

The Problem Statement
Off-road vs. on-road driving presents unique problems to radio communications. For talking to people who are trying to avoid making a wrong turn and getting lost, or needing help - reliability of communication is tantamount to performance. If I can't hear someone in the group or can't talk to them in reply, we have a potentially serious situation brewing. This is exactly the purpose behind researching the technical merits of each antenna type, as each design offers benefits and detriments to range and reliability. Ensuring that communication between 2-20 large, heavy off-road trucks and their drivers is maintained is critical to smooth manuevering and land navigation. Any impediment to clear communication must be resolved, taking into account any compromises which the system has to tolerate.

Directivity & Radiation Patterns
One of the primary attributes of antenna design, in general, is the degree to which the antenna can transmit the electromagnetic signal in the particular direction where it can be received. With an off-road vehicle forced to navigate going up and down hills, in and out of canyons with rock walls - or a road-going vehicle operating in a city environment with skyscrapers or repeaters mounted on the tops of nearby buildings - the signal strength in any specific direction is a little unpredictable. The best modeling of antenna mount location, shielding by metal of the vehicle body, along with the power & efficiency of the radio output circuits, the antenna component, and the feedline carrying the signal between will only approximate successful communication between two radios located some unknown distance apart. This describes the property of directivity in this use.

A comparison of directivity - or trying to control the vertical cone of the transmission of electrical energy across the antenna - is different and highly dependent on antenna length. Shorter length (1/4w) sends more of a 'spotlight' up the antenna vs. a longer length (1/2w or 5/8w) sending more of a 'floodlight' - to oversimplify. The net effect is that the shorter length antenna (*high angle radiator) sends a signal more up than out which is beneficial in canyons or hilly terrain. These signals will bounce and reflect, maintaining strength of the signal to other nearby vehicles. Conversely, the longer antennas (1/2w & 5/8w) feature a flatter distribution of transmission energy - a wider aperature of the radiation pattern - which is great for long distance communication over flat ground, but won't get above canyon walls. Any 'lobes' in the radiation pattern are dead spots with greatly reduced energy, and if the truck with a long antenna is moving up or down a hill and pointing those dead spots at a potential receiver - they're unable to hear or talk over the radio.

With a 1/4w antenna, the radiation pattern is smaller but more consistent, allowing for greater variation in the orientation of the antenna before dead spots become an issue between transmitters and receivers. Advantage: 1/4w.

Voltage Standing Wave Ratio (Efficiency) & Radio System Matching / Tuning
The efficiency of the total system (radio -> feedline -> antenna) is a collection of gains and losses. Even a powerful radio won't work well with a poorly designed antenna - like my 5/8w antenna with all these random dead spots trying to work off-road. The concept of vSWR (or just SWR, shortened) is that not all power is transmitted, some is lost to heat or wasted through back-feeding from a poor antenna setup. Modern HAM radios are designed to measure antenna feedback and reduce output power to prevent damage to the radio. A SWR reading higher than 3:1 is enough for your radio to cut back power transmission output. A perfect SWR reading is 1:1 with (*almost, nearly...) all power sent going up your antenna as a signal. Therefore, selecting the right antenna is incredibly important to reliable communications across a multitude of environmental conditions. While a cheap vSWR meter can be purchased for $50-60, it will really only tell you if there's a problem. To solve the problem, you'll need a more expensive Antenna Analyzer (starting at about $200+). Best bet - find a HAM friend who's more into this than you are!

So, if antenna choice is so important to overall efficiency, which one works best? Good question; as the answer is the same for either short, 1/4w or longer, 1/2 & 5/8w antennas. The shorter length whip antennas for this frequency range will be easier to tune. The longer length whips may take a little more work - but there's more there to work with. The 1/2w antenna is an interesting case, in that this particular dimension results in an end-fed voltage potential right at the peak current point - meaning that the overall length of a 1/2w antenna must be nearly perfect to resonate and work efficiently. The 1/4w and 5/8w lengths are far less picky. Advantage (*again): 1/4w.

Sidenote: Any length, ungrounded whip antenna will be resonant at exactly ONE frequency. Apart from that frequency, it can still remain very useful for radio communications. Which brings us to the next topic,

Possible Solutions;
Based on the above analysis, if we're trying to accomplish the goal of being able to talk to close-range radios 100% of the time across varying terrain where the truck and antenna orientation are likely to change in relation to the other trucks in the group and their antennas, then the 1/4w antenna keeps more of the available transmitted energy in close, where it's needed. Advantage: 1/4w.

However, if we're trying to talk over wide expanses of flat land - really trying to reach out there 40-50-60+ miles to talk to distant friends, then 1/4w may not have a flat enough radiation pattern for us to hear or be heard. In this case, we need a longer antenna like a 5/8w to help focus our available electromagnetic energy further out toward the horizon. Advantage: 5/8w.

Ideal Solution;
The perfect setup would be an A/B switchable antenna port using a 1/4w antenna for short-range, rugged terrain use which could be selected to a secondary 5/8w antenna for long-range, flatland use. While more costly - as it requires the purchase of an A/B switch, a second feedline, AND a second antenna - this presents the radio operator with the best of both worlds. Flip a switch to pick the right antenna for the job.

Compromise Solution;
If you don't want your truck to look like a porcupine rolling down the road (!), a compromise might include splitting the difference between a short, 1/4w antenna and a long, 5/8w antenna - where the 1/2w starts to shine. If we consider that the theoretically perfect antenna is a full wavelength long, then the 1/4w is missing 3/4 of the ideal, the 5/8w is missing 3/8w off of ideal, and the 1/2w is somewhere in between, is a little longer than one, a little shorter than the other, and offers performance right in the middle of the proverbial road.

So, after a few hours of researching the technical merits of the situation at hand, I'm going to roll with the compromise solution of a 1/2w tuned/matched antenna for awhile until I run into new problems needing to be solved.

Let me know if I've missed anything important, and keep in mind this is a brief analysis to keep interest...
 

sdnative

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As far as I know the 1/2 wavelength antenna does not need a ground plane (counterpoise). I drilled the roof on my 80 and used a 1/4 wave, worked great. On the 100 I am not willing to do that, and I use a rack, so I use 1/2 wave mounted to the rack.
 
Joined
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Big Mike at HRO the other day said the same, and in my estimation, the 1/2w benefits from a counterpoise, but doesn't require one when tuned properly, making it a good choice for vehicles that either don't offer a solid ground plane, or as in your case won't be drilled to create one. My bumper mount is a NMO magnetic; another situation that prevents drill through and grounded counterpoise.
 

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