Hello & seeking UHF antenna advice

[Arius007]

Hello all. I've been a somewhat irregular visitor over the years and just decided to register and seek suitable advice before embarking on my next experiment.

I've a reasonable understanding of electronics, and limited knowledge of radio.

I use a handheld Wouxun KG-UV6D, tucked inside a jacket pocket, while travelling on a motorcycle. It's on a licenced frequency ~ 450Mhz. It's used for vehicle-vehicle communication in various terrain - everything from flat to mountainous, coastal, etc. It's used with the standard, supplied antenna.

I've a problem in that the range can be limited by the placement of the radio inside my pocket. It will transmit & receive better to/from the front (for obvious reasons).

What I'd like to do is make a custom antenna, tuned for maximum gain to the appropriate frequency, and embed that antenna across the shoulders of my jacket. I expect the increased exposure will improve the range. I also expect the antenna being tuned to the frequency, rather than a broad spectrum VHF/UHF antenna, should improve performance.

I'm expecting that it will be sufficient to take a length of co-ax (I've RG-58 handy), attach the SMA connector, cut to length, attach the required antenna length (in solid copper) at the other end, seal it up, use it. What could possibly go wrong?

So, I've three questions:
1: Are my expectations reasonable?
2: Is it reasonable to expect to do this with DIY antenna construction?
3: What should I consider when building that antenna? Can it work with a length of solid copper? Can it work without significant grounding (since I don't really have anything to ground to)? At the point where the co-ax changes to single copper - do I need to do anything special apart from sealing & isolating?

Thanks for taking the time to review my query and repsond.

 

[SCPD]

The basic problem you have is that the body is a great absorber of rf.
Any antenna next to the body is going to attenuate the signal.
Any way you can put an antenna on the bike? It don't need to be a big ol' hunking gain antenna.
But a little quarter wave vertical, anywhere on the bike, is going to give you much greater range than what you want to do.

 

[Arius007]

Wyandotte, Thanks for the reply. I get what you're saying. I was going to file that one under 'Plan B'. Here's why:

All the gear is on me, not the bike. So when I step off for a minute or two, I'm not attached in any way to the vehicle. Except, that is, by my PTT cable, which I've designed to pull free at either end if I forget about it.

Which now has me thinking about using non-antenna connectors to do the same with the antenna.

 

[Arius007]

I forgot to ask... How far away from the body does the antenna need to be, to see a reasonable improvement?

And what difference will it make if I mount it on a plastic box?

Thanks.

 

[WA0CBW]

Probably a foot or more and needs to be vertical as most antennas are vertically polarized. Cross polarization (vertical to horizontal) can have as much as 10 db loss. An antenna next to the body can have a 6 to 10 db loss. Most vertical antennas are quarter-wave and need a METAL ground plane. That ground plane is the other half of the antenna. A plastic box will not work as a ground plane. Typical ground planes are at least a quarter wavelength (or larger) in diameter underneath the antenna. Non-antenna type connectors would not be of the same impedance as the antenna cable causing a mis-match between the radio and the antenna causing a high SWR possibly damaging the Power output transistor in the radio.
BB

 

[Arius007]

Ok. It seems the idea of a body mounted antenna is not going to work.

The plastic box I mentioned is, of course, a top box.

Would a coating inside, of tin foil or similar, amount to a ground plane?

I had a thought today, that I might mount the radio in the box, maybe even use the supplied antenna fed through the top, with or without an extension lead.

I'm aware that some of these ideas can seem half-baked, and I don't really want to do something dodgy. Whatever I do, it needs to result in an improved signal, but I need to consider all the other factors that can affect the usability of the entire setup, not just the radio. So don't let the crazy notions make you think I'm not interested in a real solution.

 

[Arius007]

Taking the advice above on board, I made a few changes. I made an extension cable with a few SMA-BNC adapters and RG-58 cable. I mounted the original supplied antenna on the top box, so it's now 18"-24" away from me, and I coated the inside top of the box with foil to create a groundplane (I don't know if that's adequate or not.)

So I've used it for a few days and noticed a significant improvement in performance. Thanks for the help guys!!

 

[mmckenna]

Taking the advice above on board, I made a few changes. I made an extension cable with a few SMA-BNC adapters and RG-58 cable. I mounted the original supplied antenna on the top box, so it's now 18"-24" away from me, and I coated the inside top of the box with foil to create a groundplane (I don't know if that's adequate or not.)

So I've used it for a few days and noticed a significant improvement in performance. Thanks for the help guys!!

I know I'm late to the discussion, but it sounds like you got it figured out.

I went through this many years ago trying to do the same thing with VHF radios being used on ATV's. Body absorption of the RF is a big deal. Getting the antenna out and away from the body helps a whole bunch.
The ground plane is necessary for most antennas to work properly. Compact radios like your severely inhibit performance due to the lack of a good ground plane.

By building that box, you've solved both issues pretty well. You could further improve performance by using a frequency specific antenna instead of the antenna that came with the radio. Even a flexible "portable" antenna that was specifically tuned for 450MHz would show some additional improvement.

We used to ride ATV's a lot. VHF radios were carried by all riders. Wearing the radio on the belt created the same issues you saw. I started carrying my radio in a backpack with the antenna sticking out the top. Helped quite a bit. I then added a higher gain antenna and saw even more improvement.

Sounds like you have a good set up.

 

[krokus]

All the gear is on me, not the bike. So when I step off for a minute or two, I'm not attached in any way to the vehicle. Except, that is, by my PTT cable, which I've designed to pull free at either end if I forget about it.

There are Bluetooth adapters available, for the radio, and motorcycle headsets available. That would give you a wireless option, and ability to maintain comms, when a short distance from the bike.

Sent via Tapatalk

 

[Project25_MASTR]

Public Safety speaker/mics come to mind. Especially on UHF and higher. At least for using a HT. (Surplus motorcycle radios would be even better).


Sent from my iPhone using Tapatalk

 

[Arius007]

I've been reading a bit and think I'll invest in an SWR meter and see if I can get an antenna properly tuned to 450MHz. Might as well try to get the maximum out of what I have at this stage.

Here's a question - I'm assuming my antenna is 1/4 wave. As per suggestion from WA0CBW, I've made a ground plane of a diameter that exceeds the 1/4 wave. Now, if I go to a 1/2 wave antenna, do I need a larger ground plane?

I'll look at bluetooth options too since that would be a big help, and far more reliable (i.e. less easily damaged) than wired.

 

[WA0CBW]

No, but the larger ground plane the better but you will have a high SWR. A 1/2 wavelength radiator has a different impedance (read that not 50 ohms) and will require a matching method to achieve a low SWR.
BB

 

[mmckenna]

Here's a question - I'm assuming my antenna is 1/4 wave. As per suggestion from WA0CBW, I've made a ground plane of a diameter that exceeds the 1/4 wave. Now, if I go to a 1/2 wave antenna, do I need a larger ground plane?

The benefit of a half wave antenna is that it doesn't require a ground plane to work. They do, however, work better (have more gain) when they do have a ground plane. A 1/4 wave ground plane will work just fine.

I'm using 1/2 wave VHF antennas mounted on a few UTV roll cages. They seem to work quite well in that application.

 

[Arius007]

No, but the larger ground plane the better but you will have a high SWR. A 1/2 wavelength radiator has a different impedance (read that not 50 ohms) and will require a matching method to achieve a low SWR.
BB

Is it worth the effort?

 

[mmckenna]

Might be, hard to say.
We've tried 1/4 wave, 5/8th's wave and 1/2 wave antennas on the UTV's. The limitation is the ground plane we can provide with the tubular roll cage. Running a few machines allows us to try out different antennas at the same time. Seems like the 1/2 wave antennas have worked better for us.

Kind of hard to compare a UTV roll bar mounted VHF antenna to a motorcycle mounted UHF antenna.

UHF doesn't require much of a ground plane, so it's easy to build it into the design, like you did. Lower frequencies require larger antennas/ground planes, so that changes the rules.

In other words, only way to know for sure is to give it a try. The increased gain (about 2.4dB over a 1/4 wave) might help a bit, but being low to the ground and being partially shielded by the rider might negate some of that gain.

 

[Arius007]

I've found this 14.5cm Long UHF 400-470 Mhz Antenna for Kenwood radio SMA Female | eBay which is not dual band, so I'd be expecting it to be better at 450MHz. I haven't found anything longer yet so I expect I'll have to make one.

 

[mmckenna]

I've found this 14.5cm Long UHF 400-470 Mhz Antenna for Kenwood radio SMA Female | eBay which is not dual band, so I'd be expecting it to be better at 450MHz. I haven't found anything longer yet so I expect I'll have to make one.

From the length of that antenna, it's more than likely a 1/4 wave. If it isn't, it's got a loaded design which may not necessarily work better.

Also, after market/Chinese made antennas have a pretty bad reputation. If you search around on this site you'll see many stories about this. People have taken "UHF" antennas from these sellers and put them on analyzers to discover they are often no where near resonate on the intended frequency.

Personally, I'd find a name brand, or go with a mobile antenna.

 

[Arius007]

When you say loaded, you mean like this one? Pmr Antenna Panorama Acuhb/U Uhf Ant | eBay
And you're suggesting it (loaded) might not be the way to go?

 

[mmckenna]

Sort of. The antenna you linked to earlier might be a helical design. They work OK for portable radio use, but they are a sacrifice. I'd go with a longer design if you can. A 1/2 wave should be closer to 12 inches. A 5/8ths wave would be closer to 16 inches.

Panorama Antennas makes good stuff. I had a mobile antenna from them that was pretty nice. It was VHF, but nice a flexible.

 

[Arius007]

I've no problem with 12 or even 16 inches as long as it's flexible. Am I right in thinking that the tuning is really only a matter of length? And if there's more to it, such as impedance matching, can I just do that with a coil?