base loading a 102 whip?

[mr_fx]

I am thinking about base loading my 102 whip on the mobile and setting up multiple taps for different bands... the bands I would like to setup for are, 6m, 10, 15, 20, 40 and maybe 80(just for giggles)

I have a few questions about doing this:

1.) am I on the right track?

2.) I notice some base loaded coils have spacing between the next coil, while others are almost right on top of each other... I figure the ones that are right on top of each other use emaneled wire... but other than that what is the difference?

3.) I figure 40m is doable, but what about 80m?

4.) I have looked at RF Inductance Calculator - HAMwaves.com but I am still not quite understanding what I am looking at, it's possible that it is just too early in the morning, BUT who knows

5.) as for running 6m I figured the antenna is physically pretty close to 1/2 wave, BUT couldn't I just tap it lower on the coil (between the 20 and 15m taps) that should present the same match as a 3/4 wave antenna right?

 

[W5LZ]

Try googling for 'Texas Bug Catcher' antennas. That may give you an idea or two.
- 'Doc

 

[mr_fx]

W5LZ you got me thinking

maybe this would work

4 or 5 feet or either 3/4" copper tubing OR 3/4 galvanized EMT solder in a 3/8 - 24 stainless steel bolt into a cap on both ends...

THEN... take a 3" white PVC form and install end caps and 3/8 - 24, and wrap 75 or so of 10 gauge solid copper wire I recently found at the house...

and finally a ferrule and a whip, between 36 and 48" long

then I SHOULD be able to do the following

tap for effectively for the following bands

20m, 17m, 15m, 12, 11m for giggles, 10m,

then 40m should work decent

80m should be just for giggles


but as for 6m...

the base will be too short... the top will be too short and the base/coil/whip shorted will be WAY too long

BUT I could feed it with a very small amount of coil and a matching device at the tap, and it would almost be a full 1/2 wave length ... ... ... OR I could just tap it some where between the 20m and 15m taps, that should give a decent match... but what I am having trouble figuring out is would the pattern be similar to a 1/2 or a 3/4 wave, I mean electrically it would appear as a 3/4 wave right? but it would physically pretty much be a 1/2 wave @ 6m



OTHER THOUGHTS so far: I might consider a larger fitting between then mast and the coil/whip assembly

as for copper vs Galvanized EMT

IF copper is 100% conductivity

then

Aluminum (PURE) is 67%

HOWEVER the aluminum stuff we use for building antennas is an alloy, ranging from 25-38%

regular steel is only around 2%

BUT Galvanized EMT is around 28-32% or so... it depends on the the zinc content...

but at $1.85 for 10 feet it sounds good

 

[W5LZ]

The most common method for a loaded antenna to work on the higher HF bands is that with the loading coil is completely shorted (by-passed) the length of the antenna is a 1/4 wave length. If 10 meters is the highest band you will be trying to use then the thing would be about 8 feet tall. See how that would work? Another common method of making the thing work on 6 meters would be to swap the top whip for a shorter one so that the total length of the resulting antenna with the loading coil by-passed would be a 1/4 wave on 6 meters. (The same thing is done with screwdriver antennas, BTW.)
As for figuring the conductivity of the metal used to make an antenna, it isn't worth the time and effort, just not that important at all. It's mechanical strength is much more important. If you used pure copper the antenna couldn't even stand up straight, just too weak. Aluminum tubing works just fine and also doesn't weigh much. It's more difficult to work with aluminum, harder to solder or weld, but it really does work just dandy.
As for the size of a loading coil, it just depends on what your height limits are. Short 'fat' coils tend to work better than tall 'skinny' ones. Those short-fat ones also tend not to break as easily as the longer ones. You have to keep mechanical strength in mind with all of it, all of that antenna will take a beating. If you will be running much power the spacing of the windings of the coil is the biggy. AND how clean the thing is, there's a reason it's called a 'bug-catcher'!
And you need to think in terms of a fairly large mounting method. There's a lot of force applied at that point and having a mount break can ruin your whole day. (I used a bug-catcher for years and broke several ball-mounts.)
There are a lot of factors that can be manipulated in one way of another just depending on your needs. Give it a shot, see what happens.
- 'Doc

 

[KF6IRK]

Try using one of those huge 'Trucker' coils and the 102" whip, attach a wire jumper with an alligator clip (long enough to bypass the coil) at the connection below the coil.

Connect the antenna to an analyzer and find your resonant points on the coil

 

[mr_fx]

that might be good for 15 or 20m at best

 

[northern35s]

I would consider centre loading before base loading, see if you can get hold of a second hand hamstick, it doesn't matter which band because you'll be stripping it down anyway, but if you can find a 10m version then less wire to remove.

Fabricate a large centre loading coil from copper brake pipe tubing (I assume it's available Stateside) and solder a number of tapping points to the tube for a fly lead, or use a fly lead with croc clips, here's a photo of the self supporting 5" diameter coil on my home brew multiband antenna, 10 through 20m:

If you wanted to include 40 and 80m then you would need thinner wire and a coil former, however efficiency of the antenna on those bands would be low, very low on 80m, thet's when you need to consider adding a capacitance hat, see the pic below for my cap hat:

 

[mr_fx]

just salvaged about 60ft of 10-3 copper wire (10 gauge solid core, 3 wires in one sleeve, I can build at least a couple antennas now, wire for the coil and the moxon I am building