question about CB mobile antennas (i know its not AM specific)

[ssbothwell KJ6RSG]

hi guys. as a way to introduce some of my friends to ham radio, i have convinced them to get CB rigs for their vehicles. it seemed like a fun project that would possibly result in them getting their ham licenses in the near future.

anyways.. i was reading about mobile antenna setups and what i have understood is the best place to install a mobile antenna is centered directly on the roof. anything else will impact range and propagation pattern.

i cant have a 9ft 1/4wave antenna on the roof of my truck and still fit in parking lots so i am going to have to either compromise on antenna length or antenna location. which is more important?

if i had it on the roof then it would have to be a 2ft loaded antenna with a magmount. if it was on the rear bumper it could be a 4 or 5 foot loaded antenna properly bolted to the bumper.

what should i do?

[k0vvx]

first of all you need to read up on mobil antennas for ham use. you do have a call it looks like you should know a little about antennas.

[Steve - K4HX]

That was helpful.   

Got any references or books you could recommend?

[KB2WIG]

The ARRggL  Antenna book is usefull. You don't have ta get a updated one. The laws 'o antenner physics havent changed much.


Try this site. You need to register, but its free.

http://www.cebik.com/


klc

[ssbothwell KJ6RSG]

k0vvx, i am a new ham. i do know a little bit about antennas but i have any mobile experience. i know that ideally i should be using a 1/4 wave antenna but that is not realistic in my application.

thanks for the reminder klc. i have the antenna book but haven't read the sections on mobile antennas.

choosing between a suboptimal antenna location or suboptimal length seems like choosing the better of two bad options and i was hoping to get opinions based in experience.

[Steve - K4HX]

Solomon, always remember that there are trade-offs in every antenna installation. Some common factors are size, cost, gain, bandwidth and coverage (DX vs local). You can never max out on all of these, so you play the trade-off game.

Good luck with your mobile pursuits. You should be able to make a ton of AM contacts in your mobile on 10 meters now. I heard a European mobile today.

[ssbothwell KJ6RSG]

mobile seems rather extreme with the tradeoffs. flipping through the arrl antenna book i see that antenna location plays a big factor in ground loss and antenna length impacts radiation resistance.

i dont know what the ground losses will be like but 'the antenna book' gives some sample data. it mentions rear bumper mounted antenna having a ground loss of 6ohm and an antenna mounted just below the roofline of a station wagon having 2.5ohm ground loss. these numbers wont be exactly the same as for my car but they give me a sense of what the values will be.

i guess these would be my antenna options:

2ft continuously loaded whip on roof:
electrical length (h) = 19.75
radiation resistance (Rr) = 1.25
ground loss (Rg) = 2.5ohm

5ft continiously loaded whip on rear bumper:
electrical length (h) = 49.39
radiation resistance (Rr) = 7.818
ground loss (Rg) = 6ohm

the formula for radiation efficiency is:

E = Pr / Pi * 100%

the expanded formula for radiation efficiency would be:

E = (I² * Rr * 100) / (I² * Rr + I²Rg + (I cos h1)² * Rc)

this is a pretty complex equation. can anyone help me with these calculations?

i dont know how to determine loading coil resistance. that equation involves complex impedance values that i still have not figured how to work with.

[Steve - K4HX]

A simpler formula for efficiency is

Eff (%) = 100 [R_rad/(R_rad+R_loss)]

You have numbers for ground loss. For a whip, with no loading coils, the ground loss will be most of your R_loss.

So, using your numbers for a whip on the roof, you get

Eff = 100 [1.25/(1.25 + 2.5)] = 100 [1.25/3.75] = 33.3 %

Of course, you'll need to do these calculations and include any loading/coil losses, if appropriate.

The expanded formula you posted is the same as the one I used above except it accounts for losses based on the length of the continuously loaded whip. So at best (zero coil loss) you'll have 33% efficiency.

[ssbothwell KJ6RSG]

thanks steve!

so here are the efficiencies for each antenna option:

2ft antenna on roof:
Eff = 100 [1.25/(1.25 + 2.5)] = 100 [1.25/3.75] = 33.3 %

5ft antenna on rear bumper:
Eff = 100 [7.818/(7.818+6)] = 100 [7.818 / 13.818] = 56.57 %

i wish i could put a 5ft antenna on the roof. that would give me 75% efficiency.

it looks like the 5ft antenna wins despite having worse ground loss.

would a 50% efficiency mean the antenna would have -3dB gain?

also, how much of an impact could coil resistance have on efficiency?

[W1ATR]

Hiya Solomon. CB antennas have pretty much been done to death, so there's a million choices without really having to get into the science of it all. (but it would be the best way to understand whats going on of course.)

A nine foot stainless on the rear bumper with a bit of pruning for optimum swr will yield both the best performance and widest bandwidth. There will be the slightest bit of directional gain towards the front of the vehicle, but it only works out to about an s-unit when you check it against a fixed station by driving a circle in a parking lot. In other words, it doesn't matter. The next best way to go would be with a base loaded mag mount like a wilson 1000 in the center of the roof. The bandwidth will be much more narrow, performance will be a couple s-units down from the nine footer, but it will get the job done with acceptable results. I never liked continuously loaded short antennas like the firesticks, etc, because the usable bandwidth really takes a punch in the gut, and the performance is once again, a couple s-units down from the base loaded mag mount. They are also affected strongly by any nearby sheetmetal. Use them as a last resort has always been my opinion. There are these other things on the market that are basically a center loaded, semi shortened nine footer with a giant coil in the middle that looks like someone js'd it out of a honda civic suspension. They are just for looks for the most part and totally unnecessary. Unless your gonna run 1.21 gigawatts.  

With the good band conditions, the chicken band is extremely busy, and your likely not going to be able to get out of your own neighborhood with one of those two footers you mentioned.

Food for thought.
73

[ssbothwell KJ6RSG]

hi w1atr. thanks for the tips! i enjoy learning about how stuff radio gear works almost as much as i enjoy actually using it. i probably wont crunch the numbers on a CB antenna again anytime soon but it is good to do it all at least once. this will also help when i setup an HF mobile rig in the future.

i think i am gonna go with a 5footer on the rear bumper. i dont want use a 9footer and have to worry about it hitting the garage roof!

[W7TFO]

You can always go to any local hamfest and look around the parking lot.  Ask the owners and you will get a lot of free learnin'

Wearing a hat or shirt with your call will get you more answers than not, too.

73DG

[M1ECY]

I use CB on my 4WD (Landrover)

I have a 5 foot "base loaded" springy whip, on the roof - I cant get into a lot of car parks here in the UK anyway due to the size of the vehicle, those that I can get in just end up having the whip twainging away on the obstructions.

I guess the overall height from tip to floor is about 13 feet.

CB was my way into the hobby, nowt wrong with it.

Generally when mounting an antenna on the car the radiation pattern tends to mirror the shape of the car - so in the middle, the pattern is car shaped, and evenly spread around the vehicle, rear mounted will tend to throw the signal to the front of the car, and vice versa if you mount it on the front (unlikeley).

Experimentation is the key - there is no right or wrong solution. Grounding of the antenna is important, especially on modern vehicles, where the steel content is generally lower.

Have fun, and good luck with trying to drum up interest.

Cheers
Sean

[The Slab Bacon]

Just FWIW, we have a short mag mount antenner on my wife's Ford Expolder.
If we have to go into a parking garage, it goes dingle de dingle, bang crash on EVERYTHING that hangs down from the ceiling of the garage.

If you really want to use a 9' whip you should consider either, mounting it to the bumpa, the fenda, or just stay the hell out of parking garages.   

Oh, one otha alternative would be to stick your arm out the winder and hold onto the antenner till you get parked. (been there, done that..........)

[WD8BIL]

For 10/11 meters the 102 or 108 inch whips are you best choice. No need to put up with losses in a coil on those bands.

Use a gutter clip to keep the whip "folded" down when not in use and the parking headaches go away.

[ab3al]

only one word

www.davemade.com

they wont need an real antenna wif one of these babies

[DMOD]

But isn't it mandatory to purchase the "ROD of God" and walk around with the cobra Tee-shirt?

Phil

[k0vvx]

That was helpful.   

Got any references or books you could recommend?

as metioned the arrl hand book is a good choice and if you dont have one jsut google antennas mobile hf and there is scads of articals out there that will get you the basics.

[ssbothwell KJ6RSG]

only one word

www.davemade.com

they wont need an real antenna wif one of these babies

But isn't it mandatory to purchase the "ROD of God" and walk around with the cobra Tee-shirt?

hahaha. this is hilarious. is it even legal to run an amplified cb unit?

[The Slab Bacon]

I got my 5-pill, my 5-pill, and a muddy watta slappa..................... 

[KB2WIG]

There's always that plastic WeedEater line to tie the skyhook off 'o the side view mirror.

Then there is the doglog.

http://www.amwindow.org/pix/jpg/hearse.jpg


klc

[W1ATR]

They cawl me missa wurl wide awwwwww deee oooo drive, and i jus got down break break! 

[ssbothwell KJ6RSG]

hahahhaha. you guys are killing me!

get ready for the convoy songs!

but seriously. i took a couple friends out to the swapmeet today and we grabbed a bunch of CBs and whip antennas. had some fun installing everything in one car so far. i'm gonna do another car tomorrow. atleast 5 people so far said they want to get CBs installed.

should be a fun way to get them interested in getting their tech licenses.

[Ed-VA3ES]

Being a 10M addict, I have some experience with antennas there.  CB will not be any different.

For mobile 10M use I use a Larsen NMO34  coil, (P/N NMO34B) with the Larsen  64" whip (P/N W640B) and a spring (Larsen P/N SPRINGB).  The antenna  is about 70" total length, (same as a Wilson 1000).  I use the Wilson 1000 as a  standard for bottom loaded antennas.  The antenna  works  exceedingly well, and  gives me super ground wave coverage (typically base to mobile about 40 miles and  mobile to mibile  about 20 miles more or less.  DX performance is also superb.
Now, for CB, use the Larsen NMO30 coil (Larsen P/N NMO30B), with the above 64" whip and spring.  Or, you can  go purchase a Wilson 1000.

An alternative is either a 6' or 7' helical whip.

I would mount these on the center of the trunk. Roof mounting is possible with the bottom loaded antennas,  but problematical with the longer helical whips.

[ssbothwell KJ6RSG]

after looking at a lot of mobile transciever articles and forum posts it seems like the consensus is to wire the transceiver directly to the battery a lot of people also run the transceiver ground directly onto the chassis.

why do all this? whats the difference between getting power off the stereo system's power line and getting it off the battery? and how does wiring the transceiver's ground to the battery improve the rf ground if the antenna mount is already grounded to the chassis?

edit: i guess there are issues with rf interference in car wiring but could you just put an rfi sleeve on the power and ground leads?