Age old question.. 5/8 VHF or 1/4 VHF for PS?

[SARCommCoord]

So, in a previous thread, I asked about the differences in 1/4 VHF antennas with or without a spring. In this thread, for agencies using VHF, how many of them actually use 5/8 antennas? What agencies do you know off hand that use 5/8 ?

In FL, very few agencies use VHF anymore. A few counties do, but they are covered by pretty high powered repeaters. Forestry uses 1/4 VHF, but of course they have decent repeater setups.

Part of my interest in this also has to do with a project. I do a lot of work with SAR, mainly on the medical & comms side of things. I am on a team that provides rehab & medical support for SAR teams, and recently, we have been contacted to possibly start assisting with rehab for forestry personnel on operations (through an MOU with a SAR team). Part of this project is determining what antenna to mount on support vehicles and trailer. Personally, I have a Larsen 5/8 VHF tuned for our Part 90 licensed frequency, and it covers what I need very well. I have this NMO mounted next to a 1/4 wave Low Band VHF with no issues ( mounted about 15" away due to limitations on the ground plane). For mainly flat forest conditions, what do you think would provide the best performance? 1/4 or 5/8? (part of the reason I asked about other agencies)

 

[902]

For vehicular use, especially for simplex vehicle to vehicle use, I would attempt to get as much gain as possible. Generally, the bigger the antenna, the more gain it would have.

There are some exceptions. Wider bandwidth need is usually a trade-off for gain. Operating in an urban area where users are close-in and sites are on top of very high buildings would make me change the gain antennas to 1/4 wave, as the more gain an antenna has, the flatter the vertical beamwidth becomes. That works with UHF and 7/800 MHz, too.

Low band is a different animal. The most efficient antenna for LB is a quarter wave, which is usually a ball-and-spring antenna. I see Florida people tend to mount these on tool boxes in the back of their pickups, but the best situation is to mount the antenna to the vehicle, itself. Base loaded LB antennas are a compromise of sorts. Antenna Specialists used to have a very short, really thick spring loaded LB antenna that was popular on high vehicles, like ambulances and fire trucks. These were very sharp on frequency (the antenna was basically a tuned coil), but not very efficient compared to a ball-and-spring or base loaded antenna. You won't even see these anymore unless you're looking at an antique vehicle and they are no longer manufactured.

Don't forget your mobile radio is licensed and has a maximum ERP field. If you are using a 1/4 wave at VHF, that is generally considered to be "unity gain" or the ERP is the same as the output. If you are using a 5/8 wave antenna, it is generally considered to be 3 dB over the quarter wave. So, a 50 Watt mobile would have 100 W ERP. A lot of licensees won't "fix that" on their license (and it might affect their frequency coordination), but the last answer would be "whatever kind of antenna keeps me in compliance with my license."

 

[mmckenna]

Something I'd add:

One of the beauties of 1/4 wave antennas is the wide bandwidth. With all the interoperability channels available, it would be wise to consider this. Between local, state and federal channels, you could easily be spread across 20MHz. Higher gain antennas will have an issue with that, while a quarter wave will not.

Also, the shorter 1/4 wave antennas on top of a high vehicle will likely last longer than a taller antenna that would be subjected to tree strikes.

A compromise might be a half wave antenna. It's sort of in between the two. A bit more gain than a 1/4 wave but not quite the narrow bandwidth of a 5/8ths.
A half wave antenna will give you zero dB gain if there is no ground plane underneath it, but it'll work just fine. Add a ground plane and they give you somewhere around 2.5dB of gain over a 1/4 wave.

I've been using a half wave antenna on the roll cage of a UTV for about 2 years now and I'm pretty happy with it.

 

[SARCommCoord]

So back to the drawing board, here is what I came up with. While I enjoyed the "slight" gain the 5/8 gave me in the Florida flatlands, having it tuned for around 158 limited me to 158-164Mhz. Since we work with forestry, park services etc I need at least 150-170. I have a 1/4 vhf antenna (thin gauge whip) which think I will replace the 5/8 whip with. Only issue I see is that websites show the standard 1/4 VHF only having a useable bandwidth of 10 MHz or less.

 

[mmckenna]

The 10MHz of bandwidth or less really depends on what you consider acceptable performance. If I look at keeping SWR 2:1 or less, I was seeing 144MHz to 170MHz on a few I tested. I saved the plots, I'll see if I can export them out of my tester and post them here.

 

[lamarrsy]

Hi,
If you have great distances to cover, the firing angle of 1/4 vs 5/8 may be of importance, at least equal to the +3db gain factor: a 5/8 has a lower take off angle and would provide a more "horizontal" coverage, great for long range simplex, while a 1/4 wave antenna has a steeper take off angle, that may perform better with repeaters located on mountain tops around your covered area.
I was skeptical about that but I had the occasion to test it out in real life both with the simplex and the repeaters theory: these real-life tests cleared my doubts.

Just my 2 cents!

Hope it may help!

 

[mmckenna]

Excellent point.

If you look at the VHF antennas used on top of sail boat masts they are very often 1/2 wave designs, rather than the stacked/colinears used on power boats and ships.
2 reasons for this:
1. The sailboat mast doesn't have a suitable ground plane so the 1/2 wave design is necessary.
2. The rocking motion of the sailboat is accentuated at the top of the mast. A high gain antenna on top of the mast would provide less than ideal coverage in rough seas if the energy is constantly either being shot into space or down into the water.

Sometimes lower gain antennas work better, but it's difficult for some new users to understand that. More ERP is seen as "better".

 

[nd5y]

A 5/8 wave antenna bent over when the vehicle is moving fast will have a distorted radiation pattern.

The actual gain difference between a 5/8 wave and 1/4 wave depends on how high above ground they are. See 5/8th wave mobile antenna vs 1/4 wave

 

[speedway_navigator]

>>A 5/8 wave antenna bent over when the vehicle is moving fast will have a distorted radiation pattern.

The smart in me wants to ask what happens if the vehicle is going downhill?

 

[krokus]

>>A 5/8 wave antenna bent over when the vehicle is moving fast will have a distorted radiation pattern.

The smart in me wants to ask what happens if the vehicle is going downhill?

It should be similar to the sailboat example. A lot of factors come in to play, as to whether a useful signal is presented to the receiving radio.

Sent via Tapatalk

 

[Voyager]

>>A 5/8 wave antenna bent over when the vehicle is moving fast will have a distorted radiation pattern.

The smart in me wants to ask what happens if the vehicle is going downhill?

The takeoff angle behind is higher and in front is lower. But just aiming downhill won't bend the antenna as much as wind resistance.

Two advantages of the 1/4 over the 5/8 is the bandwidth and takeoff angle.

The bandwidth was mentioned above. Those using a wide range of frequencies may need the bandwidth.

Something I didn't see mentioned is takeoff angle. In hilly terrain, you will want a 1/4 wave because the takeoff angle (the angle at which the waves leave the antenna) is greater which puts more of the signal over the hill and less into it. The same applies to reception for the same reasons.

Conversely, in flat areas the 5/8 may be better because you don't have to worry about the takeoff angle and it can be lower to the horizon which the 5/8 provides.

There is also the aspect of physical length. On larger vehicles (such as fire trucks and ambulances), you may not have the clearance for a larger antenna such as the 5/8, and the 1/4 may be your only (or best) solution.

As with many other things, there is no "clearly better" antenna for all situations.

 

[Project25_MASTR]

There is a general rule of thumb for system engineering, design everything to a standard 1/4 wave antenna. Reason being, on a properly engineered system the user will never notice a diffrence. Simplex to simplex, a very good argument could be made for 5/8 wave antennas however one has to remeber VHF-Hi is technically LOS +20% as given by the GCT.



Now, my personal preference is to use a 1/4 wave roof mounted antenna...then again I only drive full sized SUVs and pickups...even though I actually come off the caprock of West Texas.



That being said, here are some things to look at. When it comes to receiving, height is might. Here is something I've noticed. The difference in (antenna) height between a PPV Tahoe with a 1/4 wave VHF antenna and a Crown Vic with a 5/8 wave trunk mount is less than 1 inch. In most applications, you'll never see a performance difference between the two vehicles either. (Lorenzo, TX, PD has this exact same setup and use the Crosby County SO's repeater located on the other side of the county). Texas Department of Transportation uses 1/2 wave antennas on a lot of their rural trucks (they wanted the wide bandedness and as much gain as possible as some of the AO's are "dark" when it comes to regional access to the district systems and even their own county systems. Texas Department of Public Safety uses 5/8 wave antennas almost exclusively for VHF (for the same reason as TxDOT, in fact TxDOT is in the middle of a P25 migration in order to remain 100% interoperable with DPS). The main DPS sysetms operate off of 4 pairs...scattered around the state and when not in coverage of a DPS system they either use the local SO or TxDOT system.



Anyway, lots of use of 5/8 wave still out there. Is it needed? If you're in coverage area of the system you are to be using, you'll probably not need one as a 1/4 wave is probably more than adequate. However, if you are frequenting areas where you are close to the edge of coverage a 5/8th wave antenna wouldn't hurt.