Mobile Best Mobile antenna coil position

[Alexis Mercado]

Hi. I’ve read somewhere that a CB antenna with a base coil is less efficient than a center load coil, and that the center load is less efficient than the top loaded one.

According to this info. , the top loaded one is the best one ?

 

[brandon7861]

Thats true, to an extent. Where the coil is positioned is where the missing portion of antenna length is, and being there is more current toward the base of an antenna and less towards the tip, it makes sense using the coil to replace a portion of the length that carries less current, toward the top. The trade off is wind loading

 

[Dopple]

Fiberglass antennas have generally been loaded at the top. But, yeah, with whip type antennas, kinda hard to put a coil of wire at the top of it. Center-loaded would be a compromise, like the "trucker" antennas.

 

[The DB]

What you are saying is technically true, however unless the antenna is really short (i.e. noticeably less than 4 foot long) you won't notice the difference in performance based on where the load happens to be.

A word of warning. In this hobby the words "more efficient" tends to mostly be used where the difference in efficiency is so small you would never notice. When modeling a 5 foot CB antenna, the difference between a base and top loaded antennas comes out to less than 0.02 dB of difference. In the real world, this is insignificant.

The smaller the antenna the more you will notice the difference, but by the time you get to the point that you will notice the difference between base and top loads, the antenna will be so small that even in the best of cases it won't work very well anyway.


The DB

 

[Dopple]

Lots of hype in the CB market. Fancy designs. I'm using a Tram Browning 1/4 wave NMO antenna on my WK2 Jeep. Drilled 2 holes in the roof, the other one is my dual band Tram 1180. I liked the Wilson antennas for their durability, but now that they're all made in china, I'm less inclined to want to pay the premium. I've had good luck with Tram antennas, the 1180 has been in constant use since 2008, and the only failure was the bottom part of the whip. It snapped where the set screw dug into it. I replaced it with a scrap piece I had laying around, and it's been good ever since. Oh, those little o-rings in the bottom will fall out if you take the antenna on and off enough, too. :/ Now I keep spares with me.

 

[Alexis Mercado]

Hi again. Remember, I will just use this antenna for a Key-down competition only. After that, I’ll replace it with a base coil magnetic mount antenna.

I was planning to raise a trucker’s K40 antenna as high as possible. ( coil will be around 5 feet high ). Will it work the same ( or better ) than using it’s 8 inch long extension ?

In theory, should it work better ( more efficient maybe ?) or is it a waste of time ?

Some measurements:

Short 8 inches shaft , coil height close to 12.5 inches. Stinger length = 49 inches. Total length = 64.5 inches.

Long 48 inches shaft, coil height close to 53 inches, stinger length = 33 inches. Total length = 64 inches.

What do you think guys ?

 

[loosecannon]

keydown competition?

meaning you'll be sitting still?

102" whip all day long.
instead of worrying about the miniscule differences made by the coil location, why not eliminate the coil all together and have a noticeably more efficient antenna setup?

any loading coil is going to decrease the efficiency of the antenna.
LC

 

[fourstringburn]

Think of it this way…With a base load, it's like the rf currents are being choked just before it's being radiated.

A true top load is the best of the 3 types of mobile antennas while a center load is a good compromise.

The DB explained good where the actual dB losses are insignificant between all of them as far as coil design.

Any shortened mobile antenna from a 1/4 wave length will have a lowered feedpoint impedance and generally uses a loading coil to bring up the feed point impedance back to or near 52 ohms. The trade off is there are losses in the loading coil which is an inductor and the shorter the antenna, the more inductance matching is needed but then that means more losses that can be significant. This is trade off for running shortened antennas.

It's far worse for us Hams on lower frequencies for mobile antennas where on 7 mHz, are effective radiating power (ERP) using a 100 watt radio can be as low a 5 watts.

So best to run the longest antenna you can for the best ERP!

Just like Loosecannon said, ” a 102 whip all day”. Why, No coil losses!

 

[Alexis Mercado]

Thanks loosecannon and fourstringburn for your comments.

By the the way, if I get a 102 whip, can I mount it in a semi-permanent like this ?

Where do you recommend to install the 102 whip ?

 

[fourstringburn]

Thanks loosecannon and fourstringburn for your comments.

By the the way, if I get a 102 whip, can I mount it in a semi-permanent like this ?

Where do you recommend to install the 102 whip ?

If you posted a pic it didn't show up. You have to supply more information on your vehicle.

While not always possible, the center of the vehicle is best for a mostly omni directional pattern.

Also mag mounts shouldn't be used with high power due to the capacitive coupling between the magnet and vehicle. It's been known to even scorch the paint.

Provide more info on your vehicle so more advice can be given.

 

[Alexis Mercado]

If you posted a pic it didn't show up. You have to supply more information on your vehicle.

While not always possible, the center of the vehicle is best for a mostly omni directional pattern.

Also mag mounts shouldn't be used with high power due to the capacitive coupling between the magnet and vehicle. It's been known to even scorch the paint.

Provide more info on your vehicle so more advice can be given.

It is a 1999 Montero Sport. Since I don’t want to make a hole on the roof, I plan to use a lip mount on top of the the rear gate. See the picture.

 

[loosecannon]

well not to be the parade rainer, but not wanting to drill a hole in a vehicle could pretty much take you out of the running at a keydown competition.
not sure of the specific rules of the event you are attending, but most keydown vehicles have pretty elaborate antenna setups that aim the signal only where they want it to be directionally. anyone with an antenna setup like this will have a HUGE advantage over someone with a trunk lip mount and a 102" whip.

yes, you can use that mount on a 102" whip. It will be fine for sitting still. I can't speak to whether or not it would hold up while driving.

be sure to scrape the paint off of the two areas where the set screws on your mount will contact the vehicle body.
you want a good RF ground there and the paint would be in your way.

be sure to point your vehicle at whatever you are trying to talk to because mounting the antenna at the very rear of the vehicle means that most of your signal will be beaming toward the front of the vehicle.
LC

 

[Alexis Mercado]

That is the idea. I will only use the 102” whip for competition. After competition, I’ll replace the antenna for a base load Striker antenna. I’ll take the paint off to make sure there will be a good ground. I’ll only compete on a four pill category with only one antenna.

Of course I won’t compete against a 3 antenna setup rival.

Anyway, a trunk lip mount is better than a magnet mount.

 

[SuperLid]

I would guess you would want the major lobe going forward, so in the center toward the rear of the vehicle would be best. Mag mount is good too, but you need to attach copper/plated/tinned braid to the mount and the vehicle. I have been doing this for years, with multiple braids, to each side of the vehicle, using small screws with star washers in the rain gutters.

 

[ForestRunner98]

Just curious and you (OP) don’t have to answer but how much power are you going to be running?? It’s a key off and that usually means crazy big antennas, monster boxes and a zillion alternators.