WeirdJim - HAM radio question..

[Doggtyred]

Over the past few years I've gotten into HAM radio use (motorcycle mounted, used for volunteering at charity bicycle events).. and I've slowly managed to learn a bit here and there.

We operate on 2m (144mhz) and 70 cm (440mhz) bands. Given the nature of many motorcycles, its difficult if not impossible to get an effective ground plane. The solution, I've been told, is to use 1/2 wave antennae.. and the dual band antennas that we use are 1/2 wave for VHF/2m and 2 x 5/8 wave for UHF/70cm..

Now is where my question is coming from. While surfing the net looking for ways to improve my installation, I came across an article regarding 1/2 wave CB antennas.. and how with the 1/2 wave CB antennas they come with a tuned length of coax (in the neighborhood of 13 or 17 feet... in any case, the instructions say coil it, dont cut it...)... Cutting the coax to fit/convenience with that 1/2 wave antenna is supposed to screw things up signal wise (at least thats how im trying to wrap my head around it)..

I do realize CB is way down the spectrum from 2m/VHF and 70cm/UHF, but is this a consideration? I am planning on putting a 1/2 wave antenna on the butt end of a harley tourpack and my coax run could be measured in inches, not feet.... if the length of the coax is not critical to operation.

Do you know if the coax length is a factor here or not?.. and how to calculate would that length would be (or be really nice and just tell me how long..)?

Given your decades of experience with antennas and radios I'd figure I'd ask you first before plodding through dozens of amatuers out in my neck of the woods..

 

[steingar]

I wouldn't divert my attention from the road to talk to anyone. Too easy to get dead on a bike. My $.02 worth, which is worth even less than that.

 

[wsuffa]

It comes down to impedance matching and power transfer. The coax is operating as an impedance transformer to convert the base impedance of the antenna (which at a half-wave is NOT 50+j0) to the impedance the transmitter wants to see. The impedance transform of that matching section is based on the length and velocity factor. (there's also a phase transform, but that is unimportant for a single, unidirectional monopole antenna element).

Cutting the coax shorter affects the impedance transform - the length is important to get a correct match at the antenna.

So, yes, length is important, as is the true length of the antenna in wavelengths and the type of coax (for velocity factor).

Do you have any experience with Smith Charts or method-of-moments calculations? They're supurb for solving this kind of issue.

 

[Doggtyred]

No experience whatsoever, Jim.. The closest I've come to any of this in the past is measuring out copper foil to make an antenna. Look up freq.. mark with ruler.. snip..

I just googled and wiki'd both terms.. and got a little glassy eyed.. I'm a smart guy but RF engineering is not something I've delved into heavily... If there IS a chart that has everything on it where i can drag my finger along and pick X marks the spot I'm sure I could figure it out..

The Antenna I'm getting is 40" from mount to tip, is advertised as a 1/2 wave for 2m/144 mhz. Anticipate using RG-58 and UHF couplings..

 

[Palmpilot]

Excellent answer from Bill S.

By the way, it's "ham" radio, not "HAM" radio. The latter implies an acronym, which it's not.

 

[twdeckard]

the way, it's "ham" radio, not "HAM" radio. The latter implies an acronym, which it's not.

But it is an acronym:

It was the call sign for the first wireless radio station operated out of Harvard. It was started by three guys named Hyman, Almy and Murray. Their call sign was HY-AL-MU which was shortened to HAM.

I'm not lying on that one, but I may not be telling the truth ...

 

[wsuffa]

The Antenna I'm getting is 40" from mount to tip, is advertised as a 1/2 wave for 2m/144 mhz. Anticipate using RG-58 and UHF couplings..

Does it have a matching section already? Is there any data on the input impedance? Which make/model?

I'll see what I can do - some of my old engineering stuff needs to be unearthed....

 

[Palmpilot]

But it is an acronym:

It was the call sign for the first wireless radio station operated out of Harvard. It was started by three guys named Hyman, Almy and Murray. Their call sign was HY-AL-MU which was shortened to HAM.

I'm not lying on that one, but I may not be telling the truth ...

Well maybe all the dictionaries and the ARRL are wrong when they show it uncapitalized.

http://www.arrl.org/catalog/?item=9639#top

 

[Doggtyred]

No... amatuer antennas are sold separately from the coax.. and I've never seen packaging inserts regarding the subject matter.

Antenna is a Diamond NR770HA - its on order but not here yet.

As a matter of fact coax length wasn't even a concern to me until I came across the issue regarding motorcycle CB antennae while trying to hunt for a good motorcycle amatuer antenna.

Responding to the poster about talking to others while on a bike.. I hear ya.. BUT.. the situation is.. 90 motorcycles working a line of 13000 bicyclists on 2 lane roads in the country.. its too congested and too slow for any cars coming up the road to get above 15-20 mph.

Most of my transmissions end up being while stopped on the side of the road at an incident or accident. The limit of what I tend to say rolling down the road is "enroute" or "on scene".. using a handlebar mounted PTT and integrated intercom.

 

[wsuffa]

Antenna is a Diamond NR770HA - its on order but not here yet.

As a matter of fact coax length wasn't even a concern to me until I came across the issue regarding motorcycle CB antennae while trying to hunt for a good motorcycle amatuer antenna.

I looked up the antenna. It's a dual band antenna with a coil in the middle of the element.... and a matching section at the base.

I wouldn't bother with specific coax lengths for use in the ham bands. There's no need - this antenna has a matching section designed in.

Install and go

 

[Doggtyred]

I looked up the antenna. It's a dual band antenna with a coil in the middle of the element.... and a matching section at the base.

I wouldn't bother with specific coax lengths for use in the ham bands. There's no need - this antenna has a matching section designed in.

Install and go

Explain what that is/how that works to a guy who doesn't have an EE degree, por favor... the coil and the matching section...

and thanks for the short answer and green light.
Dave

 

[wsuffa]

Explain what that is/how that works to a guy who doesn't have an EE degree, por favor... the coil and the matching section...

and thanks for the short answer and green light.
Dave

Ah, it's for impedance & phase matching. Basically an antenna element presents, at it's base, a drive point impedance (resistance + reactance). You average amateur transceiver antenna jack is designed to operated into a 50 ohm resistive load (reactance = 0). (In real life the transceiver will not be exactly 50 ohms, but for all practical purposes it's close enough). Maximum power transfer occurs when the impedence of the antenna, feedline, and antenna are all the same.

That's why we use 50 ohm coax cable.

So the Diamond antenna has a coil in the middle to serve the purpose of creating 2 stacked 5/8 wave antennas on the 440 MHz band while allowing the full length of the antenna to operate at 1/2 wave for 146 MHz. It serves to alter the phase of the 440 MHz signal to make the top half and bottom half work together (adding gain at low elevation angles and taking energy from angles straight above the antenna). The coil has some effect at 2 meters, but not much... it makes the antenna element look a bit longer. That's how they combine 2 bands in one antenna.

As for the matching coil at the base: the vertical radiating element has some impedance (resistance + reactance) at the bottom. For our purposes, the exact impedance of the antenna element alone doesn't much matter - Diamond has measured it and determined the value. The matching coil transforms the impedance of the antenna element to something close to 50 ohms resistive at the jack at the bottom of the coil.... the jack you attach your coax to.

So we have a transmitter that wants 50 ohms, 50 ohm coax, and a nominal 50 ohm input to the antenna system. Maximum power transfer should occur.

 

[Ghery]

If the antenna is tuned to provide a 50 ohm load, the length of the coax is immaterial. Well, the longer it is, the more loss it has. And it does have to be long enough to reach from the radio to the antenna. Other than that, it doesn't matter. I remember CBers who were convinced that the length was important for other reasons, but they were wrong.

Check the ARRL Handbook or the ARRL Antenna Handbook for more details than you care to read. And, if you start having interference problems, the 2nd Edition of the ARRL RFI Handbook is a good place to start. Chapter 15 if your computer is involved.


73

Ghery, N6TPT
ARRL TA

 

[wsuffa]

If the antenna is tuned to provide a 50 ohm load, the length of the coax is immaterial. Well, the longer it is, the more loss it has. And it does have to be long enough to reach from the radio to the antenna. Other than that, it doesn't matter. I remember CBers who were convinced that the length was important for other reasons, but they were wrong.

Check the ARRL Handbook or the ARRL Antenna Handbook for more details than you care to read. And, if you start having interference problems, the 2nd Edition of the ARRL RFI Handbook is a good place to start. Chapter 15 if your computer is involved.


73

Ghery, N6TPT
ARRL TA

The problem, Ghery, is that some of the whip antennas used in the Criminals Band radios were anything but 50 ohms. Certain lengths did make a difference. But in general we agree.

-.. . -.- -.-. ..--- -.. .-

 

[Ghery]

The problem, Ghery, is that some of the whip antennas used in the Children's Band radios were anything but 50 ohms. Certain lengths did make a difference. But in general we agree.

-.. . -.- -.-. ..--- -.. .-

There. Fixed it for you.

And I had one guy tell me that you had to use a 1/4 wavelength of RG-59 between the radio and the illegal linears he was selling. Never could get a straight answer as to why that was the case. A lot of misinformation floating around in that world.

 

[wsuffa]

There. Fixed it for you.

And I had one guy tell me that you had to use a 1/4 wavelength of RG-59 between the radio and the illegal linears he was selling. Never could get a straight answer as to why that was the case. A lot of misinformation floating around in that world.

I'm sure he didn't know.... but if I had to venture a guess I'd say that the input to the leeen-e-are was anything but 50+j0.

 

[Ghery]

I'm sure he didn't know.... but if I had to venture a guess I'd say that the input to the leeen-e-are was anything but 50+j0.

Very likely. An easy problem to fix, however. But, RG-59? That's 75 ohm coax. Oh well.... That was over 30 years ago.