Radio Shack HF vertical kit

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nanZor

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Got the urge to convert a CB antenna down to 14 mhz or more, so thought I'd put it in kind of kit format. It is designed to be very lightweight and flexible in configuration - suitable for indoors, patio, or even permanent with adequate tie-downs.

The short story: Take a cb antenna, and top-load it to lower the resonant freq with light-duty components.

Here is a shot of the base, and the antenna. It consists of

#33-370 microphone stand
#21-937 mirror / rack mount with backing plate turned vertically.
#21-988 CB stainless whip with loading coil, 64 inches tall
Optionally use the #21-903 stainless whip with no coil. I chose the smaller whip for easier management indoors near windows, etc.
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nanZor

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Optional: bypass the coil

I wanted the short whip, but plan to use only top-loading to lower the resonance as will be shown later. It is easy to bypass the coil with a short jumper as you screw the antenna down into the mount threads.

If you do want to be able to make the coil insertion optional, don't leave a dangling jumper. Remove it completely. In this case, perhaps just a small lug on the bottom would be ok as a connection point for a jumper when you do decide to bypass it again.
 

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nanZor

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Choking the feedpoint:

Unless you want your coax feedline to become part of the radial system, it is best to choke it near the feedpoint. There are many ways to do this, although RS has some suitable chokes that will work.

I recommend no less than 4 pieces of #273-104. They come in 2-packs. Using 6 of them stacked would be even better.

There are many sources of ferrites, but this will suffice if you have to build it tonight. :)

NOTE: RS also stocks convenient #273-105 snap-on coax-cable chokes, but they are really not the right mix of material for HF.

Air-core coil-wound "Ugly Balun" also works. One source for the diameter and number of turns can be found here. Be sure to wind enough turns to cover the frequencies of interest.

Common-mode chokes

You can put them in a simple stack like this - I was able to get about 3 turns inside - if you can, try to squeeze in as many turns as you can without pinching or deforming the coax. This was the hardest part of the project!
 

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nanZor

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Radials:

This was pretty simple. Normally one runs 1/4 wavelength radials out. You need at least one. Generally more is better. Run them on the ground, or elevated. Radials have been discussed elsewhere, so this is really just a short construction note.

What I will say, is that when we get to top-loading the antenna itself, these radials if elevated will usually be much shorter - about 1/8 wave long, which is also great news for the indoor enthusiast.

Note that this antenna is not directly connected to ground, ie no jumper to a ground rod, although you can certainly do that if you want.
 

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nanZor

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Top Loading to lower resonance

Here is where the real fun starts. Essentially we are just capacitively loading the top of the antenna (as opposed to unductively loading the bottom or middle).

The simplest "capacity hat" if you will, is just two wires attached to the tip, and run away from each other horizontally. Generally, the "missing length" of what would be a normal quarter wave, is just multiplied by two for the total length of the horizontal load. Use more wires, say 4, running into and out of this page, and the resonance goes even lower. OR, you can use more wires, but they can be shorter.

Again, this is more of just a quick note, and not a real treatise on the topic.

Here is the simple connection with two wires run out horizontally at the top. Each side is 5 feet long for this quick view. You can do better than I can with the alligator clip. :)
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nanZor

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Top Load variations

You don't have to have the top load wires perfectly horizontal. This is good news if you don't have the supports.

Although the loading is more efficient horizontally, you can also run them as a "Y" shape. Indoors you can run them to the ceiling perhaps. Use of blue painter's tape and light-gauge wire will get you through the weekend at least. Outdoors, the top-loading wires help stabilize this fragile mounting system. :)

Note that if you do transmit through this, allow for sufficient spacing at the ends since there is high rf voltage present - don't burn anything down or start a fire in your attic without spacing the ends aways from the supports.

Also, arranged symetrically, the top load wires do not contribute much at all to radiation or reception. They merely serve as capacitive wires. However, if you were to leave off one top wire, it would be more like an inverted-L antenna. This WOULD give you some high-angle radiation - the composite sum of the two fields (not two distinct fields), but at this point it is not truly a "top loaded" antenna ... moving on ...
 

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nanZor

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Sloping top-loading wires.

You can also run them downwards as well, but again, it will not be as efficient as horizontal. Try not to go beyond 45 degrees. If your top load wires are long, try not to get them too close to the ground.

Getting as horizontal as you can is a good idea. Although, in the ON4UN book on low-band dx'ing, there is mention that even with the sloping top-load wires at this extreme angle, you may end up being 3db more efficient than a base loaded coil - so do not be afraid to experiment.

And, I do not recommend actually using these things right up against walls or other surfaces! This was done just for taking the pics.
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nanZor

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Wrap up:

I run larger versions of these things. Certainly the 102" whip is preferred if you have the room. Of course you could make this whole thing out of tubing or wire if you wanted.

There is a lot of room for play and experimentation. One thing I do is use an MFJ mobile tuner for 160/80m with this. Although, with band noise levels so high at times, even that isn't necessary. If you get this antenna down to 13 to 18 mhz, it makes an ok general purpose vertical. And of course all of this is useful to the SWL.

Having an antenna analyzer handy makes short work of things. However, the biggest contribution is by using the top-loading wires to get the resonant freq down. Being able to use or bypass the coil is handy too - but the taller you can make this thing the better generally.

The publications of Les Moxon, G6XN and Rudy Severns, N6LF, in addition to John Devoldere, ON4UN and L.B. Cebik W4RNL, got me hooked on top-loading anything I can get my hands on. I can't thank them enough and wish I had paid attention to it sooner.

Hope this helps anyone who wants to put together a simple vertical out of parts that they may already have laying around, or needs a project-fix right away. :)
 
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nanZor

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Suggested starting point

To get down to about 14-18 mhz, some suggested lengths as starting points:

64" Whip with coil:
Attach a 6-foot top loading wire to each side of the vertical top. (12 feet horizontal total).
Leave coil inline
Attach two opposing 8 foot radials.
To easily move higher up the band - short out coil with jumper

102" straight whip:
Attach 8-foot top loading wire to each side of the vertical top. (16 feet horizontal total)
Attach two opposing 8 foot radials

Below 14mhz, a tuner may come in handy, unless band noise / static crashes at night for example, make the need questionable.

Above 23 mhz or so, the antennas when top loaded are too long electrically. Reduce the size of the top-loading wires and/or the radial lengths. For the 64" whip with coil, do not even use the coil. Or build a second one!

Notice I am dealing with only two top loading wires and two radials - typically elevated a foot or more above the ground. If the base of the vertical is practically ground mounted (but not connected physically to it), try raising the ends of the radials a foot or more above ground at the end. You can also do a "gulllwing" configuration of the radials by bringing them up a foot at a 45 degree angle, and running the rest horizontally.

Forgot to mention that once you put on a set of top-hat wires, it will essentially cut off anything above them - in other words, keep the top loading wires at the top of the vertical, and not anywhere below.

Many, many ways to do things. All of them fun hopefully.
 
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nanZor

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Got the mirror mount at the same place - RS. There is better hardware out there for sure, but this might be just the ticket for a last minute field-day run. :)

Speaking of which, many of the SO-239 connectors on these mounts are smooth without any notches in them, yet all of my pl-259's have at least two small ridges. That means that even if you screw things down tightly, the connection points are just the shell threads, and the two small ridges. If I were transmitting, I'd think about either notching the mirror-mount connector, of filing down the ridges on the cable connector for a better connection.

For the radial connection, make sure to connect them to the main plate and not the screws on the backing plate, otherwise you are going through a high-resistance screw.
 
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nanZor

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Upgraded the top-load connection. Shown without the top-load wires.

Got this from the electrical department at the home-improvement store. Basically just a bus-bar connector. There is a lot of metallic incompatibility here going from stainless to aluminum to copper. So it's not the best idea for permanent installs.

Fortunately the RS whips don't have huge end-caps, so that makes this a lot easier.
 

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nanZor

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Beefing up the mount:

Upgraded to the extendable floor-mic stand #33-340. Handy for getting the radials elevated. Shown it is fully extended, although with the 102" whip, I'd collapse it down half-way. Luckily the top-load wires also serve to guy and stabilize things.

The mount is also useful indoors with the smaller whip. Not shown are the radials and the feedline.
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nanZor

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Some stealth and portable notes

Since the top-loading wires are serving as capacitors for the most part, and not really meant to receive or radiate, they don't have to be out and in the clear. You can put them underneath an overhang or some other structure - although if transmitting you do need some safety margin.

If you are using the small whip and the overhang is a few feet higher, you could run the top load wires under the overhang, and let a drop wire come down from the center and attach it to the smaller whip.

Also, if you run out of room for the top loading wires, you can run out just as much as you can horizontally, and then let the ends drop down for the last couple of feet, much like you would do for a limited-space dipole. It starts to look like a "bobtail" vertical antenna at that point. The key is to try and keep them symetrical. This might be useful in a cramped attic.

Stealth - if you have no choice but to run without any radials, you can try moving the feedpoint ferrite chokes back about 1/8 to 1/4 wave away. This isn't perfect, and is probably the only time I'd recommend moving the chokes away from the feedpoint and purposely trying to use the common-mode. Well, perhaps even if you are using only one real radial, you can now run the feedline away from it in the opposite direction and try to choke the feedline again 1/8 to 1/4 wave away. To do this right, you need a LOT of choking impedance. Maybe the best option is to just put out as many smaller radials as you can if you can get away with it visually.

So far, mine have been working well. Since the bands are in really bad shape right now, I'm not getting fooled by good propagation.
 
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nanZor

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Thanks - if one has the supports, all this can be done with wire runs. Top loading / end loading is nothing new. You could even do this at the end of a sloper, horizontal dipole, ends of an inverted vee etc.

I don't know how much power the coil in the small whip can take - the only rf reaching these for me comes from a Comet antenna analyzer, so if you do use it, qrp is probably the best bet. Even better is to bypass the coil completely, and use a longer run of top loading, which is more efficient.

I don't know what got into me - I guess I was in the store and just had to build something. :)
 

LtDoc

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About those top loading wires.
They definitely will affect the antenna since they are a part of the antenna. If that antenna with the loading wires is used for transmitting then they will also definitely radiate. All parts of an antenna radiate, some just seem to radiate more than other parts. (Non-conducting parts don't radiate but if it's a conductor and if it's attached to an antenna, why would you expect it not to radiate or -affect- radiation?)
Almost anything can be used for an antenna. The only requirement is that it conduct electricity and have characteristics that combine to make it at least partially beneficial. That does not mean that whatever the thingy is will be a -good- antenna, just that it will 'work'. If that 'working' satisfies the users requirements, then it's a 'good' antenna for that particular user and situation.
- 'Doc
 

nanZor

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About those top loading wires.
They definitely will affect the antenna since they are a part of the antenna. If that antenna with the loading wires is used for transmitting then they will also definitely radiate. All parts of an antenna radiate, some just seem to radiate more than other parts.

If they are run more or less symetrically, current divides equally and the fields cancel, essentially leaving only the capacitance to do anything useful. However, when transmitting, low current in the middle usually means a higher voltage at the ends, hence one must space it away from conductors for transmitting. This is one reason you can hide the top loading wires more or less if you want to for rx-only.

The Inverted L is another story. Consider this a "one legged top loading wire". With no symetrically opposing wire to divide the current and cancel the radiation, you essentially get the vector-sum of the two fields, whereas the T-antenna loading wires do the cancellation - if they are symetrical.

One thing I did today was put some length between the bottom of the coil and the radials. I ran a 1 foot jumper down, and use the end of that as a common attachment point for the radials, instead of going directly to the mounting plate. We'll see. I know it is not a good idea to put a capacity hat right on top of an inductor in a mobile antenna, so I'm going to do more research and see if this applies to putting some separation between the bottom of the loading coil and radials.

The Comet analyzer shows that with two 6 foot top loading wires, and coil in place, along with two 8-foot radials, I'm smack dab into 20 meters.
 
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nanZor

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T-Vertical current plot

Here is a reason why I like top-loading so much. There is minimal current in the top loading wires, enough for me to basically ignore. Even with a very short vertical, the current is equally distributed along its length due to the top loading, rather than being concentrated down low as it would be on a base loaded coil.
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nanZor

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Speaking of which, many of the SO-239 connectors on these mounts are smooth without any notches in them, yet all of my pl-259's have at least two small ridges. That means that even if you screw things down tightly, the connection points are just the shell threads, and the two small ridges. If I were transmitting, I'd think about either notching the mirror-mount connector, of filing down the ridges on the cable connector for a better connection.

This problem just bit me over the last few weeks of high temps - I didn't file down the notches, nor create any in the mount so guess what? I was attributing super weak signals to bad propagation / flares, and when I finally got around to checking the connector, it had worked just loose enough to lose good contact with the shield! ARgh!
 
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