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1/4 wave vs. 5/8 wave

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freema22

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I have some questions that I have never really attempted to solve myself. Two years ago, I got a new truck and had a pre-tuned, Antennex 1/4 wave antenna mounted on the hood of my Tahoe. I always got complaints about the quality of my signal when out on calls in an urban setting. I put a Diamond amateur dual band antenna on my truck and tuned it for our channel (155mHz range) and things improved slightly. One of our resident radio gurus said that amateur equipment was not very durable and I would probably see performance start to drop off after a while. Well, here we are 2 years later and I am noticing a performance drop from where I was. I haven't put a meter on my rig, but I do notice that reception is not like it was before. Here are my questions:

1) Is there really that much difference in performance between 1/4 and 5/8 wave antennas?

2) What about the comments from the radio guru about the quality of amateur equipment?

3) What would explain the performance drop off that I have noticed?

4) Should I pull out the old quarterwave and try to tune it for my current setup?

5) I am using a hood mount because I have no other choice (overhead clearance issue). Is that setup really as directional as some suggest?

Thanks in advance for the suggestions.

Mike F
 

DPD1

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5/8 typically has a lower angle to the horizon, which is part of what accounts for better performance. I'm not sure I understand what the comment about amateur equipment means... There's cheap and good in any equipment, and most basic mobiles are fairly the same. You would be better off if it was on the roof. Maybe you could get the kind that has a fold over hinge. First I would take the one you have apart and clean all the contacts. Sometimes the contact between the mount and antenna gets corroded or loose, and even though it seems tight, it's not. If the one on the hood is on the side, then yes, it can distort the pattern.

Dave
www.DPDProductions.com
Antennas & Accessories for the RF Professional & Radio Hobbyist
 

n5usr

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I have amateur antennas that are 15 years old, and work as well today as the day I bought them. Unless you get the high-gain models with all the flimsy loading / phasing coils in them, there's very little that can go wrong with an antenna. Now, there are a fair number of amateur antennas (again, usually higher-gain models) that can't handle as much power and might be pushed pretty hard by a high-power (110W) commercial mobile. But a 5/8 wave antenna in particular is a pretty basic, and very durable antenna.

If you are having lesser-quality receive now, I would be looking at the mount and the coax. I assume by hood mount you mean one of the L-brackets attached to one fender? I use those too, and with most there is relatively little standing between water (particularly wind-driven rain while driving) and the coax on the underside of the mount. It's possible that over time you have had some water get into the coax, which will cause it to deteriorate. Or it may have gotten pinched somewhere on the way in to the radio.

As was mentioned before, a quarter wave antenna radiates more-or-less in all directions, including almost straight up. The 5/8 wave compresses everything into a bit of a doughnut shape, which means a stronger signal on the horizon at the expense of up high. Normally, that's a better thing, but in some cases - notably if using a repeater that's mounted very high above the area - the quarter wave with higher angle radiation can be better.

With a hood-mounted antenna, you have a non-ideal ground plane as well as the shadowing of the roof. It's quite possible you would see better performance with a 1/4 wave centered on the roof than you do with a 5/8 wave on the hood - and it could well be short enough to fit in the garage as well.
 

N1GTL

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freema22 said:
5) I am using a hood mount because I have no other choice (overhead clearance issue). Is that setup really as directional as some suggest?
If by hood mount you mean an "L" bracket on the fender, did you load up the underside with silicon caulk? If it is an NMO mount, those are really made to go thru a body. An "L" bracket is not the intended use for those mounts. You get little, if no, ground plane and the bottom is not weather tight. It may be possible that prolonged exposure has allowed water to periodically get in and some corrosion may have built up.

And yes, it will make your transmissions somewhat directional.
 

freema22

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The mount I am using is the Diamond K400. All of our operations are in an urban setting (metro Chicago) with virtually no repeater ops. Is the 5/8 wave ideal for this setting?

Mike F
 

freema22

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n5usr said:
With a hood-mounted antenna, you have a non-ideal ground plane as well as the shadowing of the roof. It's quite possible you would see better performance with a 1/4 wave centered on the roof than you do with a 5/8 wave on the hood - and it could well be short enough to fit in the garage as well.
There is definitely no option for roof mounting. I only have 2" of clearance at both my house and the airport (my home away from home).

Mike F
 

n4yek

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freema22 said:
.....I put a Diamond amateur dual band antenna on my truck and tuned it for our channel (155mHz range) and things improved slightly.......


2) What about the comments from the radio guru about the quality of amateur equipment?

Thanks in advance for the suggestions.

Mike F
All other suggestion by the others pretty much agree with each other as with what I would tell you. But to answer question number 2 slightly different here is my opinion.
A Diamond amateur dual band antenna is made for 2 ham radio frequency bands and had a coil in it's rod for multi-band purposes. When you cut it to tune it for 155MHz, you could actually be hurting yourself since you have moved its designed operating range by 15 MHz. With the coil still in line, there isn't anyway to tell what the radiation pattern is or how the loading coil it affecting the transmitted RF power. The coil could actually be eating up your power even though it has a good SWR match to the radio.

Just a suggestion.
 

freema22

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n4yek said:
All other suggestion by the others pretty much agree with each other as with what I would tell you. But to answer question number 2 slightly different here is my opinion.
A Diamond amateur dual band antenna is made for 2 ham radio frequency bands and had a coil in it's rod for multi-band purposes. When you cut it to tune it for 155MHz, you could actually be hurting yourself since you have moved its designed operating range by 15 MHz. With the coil still in line, there isn't anyway to tell what the radiation pattern is or how the loading coil it affecting the transmitted RF power. The coil could actually be eating up your power even though it has a good SWR match to the radio.

Just a suggestion.
I wondered about the coil. Before I bought the antenna, I asked Diamond for guidance on tuning it. They just told me to start cutting in 1/4" increments from each element. They didn't mention anything about the coil. What can I do there?

Also, I took the antenna off of the NMO mount today to inspect the mating surfaces. They had turned black with oxidation (brass). I took some sandpaper to the antenna and the mount to make them shiny again. I can't imagine all of that oxidation could have been helping anything.

Thanks for the help.

Mike F
 
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n5usr

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I run primarily simplex here in the OKC metro area. A 5/8 wave does work better than a 1/4 wave for that, particularly since our terrain is fairly flat.
 

n4yek

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freema22 said:
I wondered about the coil. Before I bought the antenna, I asked Diamond for guidance on tuning it. They just told me to start cutting in 1/4" increments from each element. They didn't mention anything about the coil. What can I do there?

Also, I took the antenna off of the NMO mount today to inspect the mating surfaces. They had turned black with oxidation (brass). I took some sandpaper to the antenna and the mount to make them shiny again. I can't imagine all of that oxidation could have been helping anything.

Thanks for the help.

Mike F
As far as the coil is concerned, there isn't much you can do about it. Its design is for Ham radio purposes and nothing more.

N5USR stated he has good results with a 5/8 wavelength antenna for his use and he is also in a metro setting. I would try his suggestion and get one for the frequency range you need, not one for ham radio and trying to re-tune it for your band. The price range should be about the same.

As for oxidation, nope that definitely wasn't helping you.
 
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oldranger

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1/4 Wave vs.5/8 Wave

1/4 wave antennas are direct fed and have what is considered unity gain or a gain of 1.
5/8 wave antennas are supposed to be shunt fed (coil) and the gain is arround 3 db.
Fellas, I didn't make this up.I got it out of an old ARRL Antenna Handbook.

Mike,Know Code Tech ,WB4LFC
 

key2_altfire

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freema22 said:
2) What about the comments from the radio guru about the quality of amateur equipment?
It's It's conceivable. In a commercial setting where the radio sees lots of use, it will be exposed to more severe heat cycling, which will accelerate the aging process of the transistors, which will change their frequency response and gain. I work for a company that builds commercial and military RF amps, and most of the RMAs we get are older units that aren't able to maintain the gain they once did.

A larger, "heavy duty" radio will have more heatsink mass and more sophisticated RF transistors and should maintain its tune through more use and abuse.
 
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