The "all-thread" 1/4 wave antenna project

[loosecannon]

hi all,

just thought id post on my latest antenna homebrew project in case anyone else wants to build one.

i was looking for a 102" whip style antenna that i could tune, and it had to be something that i can take apart and put in the 6' bed of my truck.

this antenna is not for use while in motion! its for sitting still DXing in my truck.
something i spend a fair amount of time doing these days due to my job.

it also had to be cheap, and the materials easily available. (so others can try it)

here are the materials i used: (all purchased from homedepot)

1. 72" length of 3/8 x 16 all thread. (make sure its straight)
2. 10' piece of 1/2" EMT conduit. (the thin wall stuff. make sure this is straight also)
3. 3/8 x 16 coupling nut (approx. 2" long)
4. 3/8 x 24 x 1" hex head bolt
5. 3/8 x 24 nut
6. 3/8 x 16 nut
7, 3/8 flat washer and 3/8 lock washer

total cost was under 11.00

first i cut the conduit to 48". then i ground down the corners of the 2" coupling nut until it was basically rounded. only do this to one half of the coupling nut. leave the other half as it was.
once the coupling nut would fit nicely inside the conduit, it was welded in place.
this left about half of the coupling nut sticking out above the conduit.
this is the end where the all-thread is screwed in.

next i prepared the base assembly by first grinding down the corners of the head of the 3/8x24 bolt so it would fit inside the conduit. then i put a 3/8 washer on, followed by a lock washer, followed by a 3/8x24 nut.
this was tightened securely.

next the head of the 3/8x24 bolt was inserted into the open end of the conduit and the conduit was welded to the washer.

now you have a 48" length of conduit that can be screwed in to any 3/8x24 mount.

next a 3/8x16 nut was threaded on to the all-thread so that once correct tuning is achieved, the nut can be cinched down against the coupling nut, thereby keeping the all-thread in place.

the all-thread is then threaded into the coupling nut until the desired length is achieved. (approx. 108" for me)

thats about it! i do want to add some sort of corona ball on top, but that will come later.

the antenna i built should tune from 24-30mhz and beyond.

i have not gotten to do any testing with it yet, as i barely got done before work started.
tomorrow i will do some testing and post the results.

the only issue i can see possibly being a problem is the shape of the all-thread itself.

because of skin effect, im not sure what effect the "jagged" outer shell of the all-thread will have.

anyone who is knowledgeable about such things is highly encouraged to post here to put my mind at ease. LOL

oh, and yes, my mount is on the roof of the cab of my truck, and it does have a backing plate added for extra support.
thats definitely something to consider when using this antenna on a windy day!

LC

 

[W5LZ]

Makes sense to me, and I don't see why it wouldn't work just dandy. I have no idea how that threaded surface of the thing will affect 'skin-effect', but really doubt it it'll be a huge problem of some kind.
Corona ball? How about a couple of nuts to fit that all-thread, screw'em onto the top. 'Round' the edges off a bit if you want to.
- 'Doc

 

[Shockwave]

It depends on if it's fine threads or course threads. It's been shown that course threads produce 20% more audio then fine threads. The course threads slow the RF down by reducing the velocity factor. Giving the radio more time to apply modulation before it radiates the signal, thereby increasing the audio duty cycle. Fine threads just act like a big loading coil offering tons of inductance to the skin effect unless coated with a generous amount of snake oil. OK, enough joking around, the threads do nothing detrimental to the antenna. I was just having fun. You've done yourself a fine job at building your own adjustable 1/4 wave antenna from available parts.

 

[HomerBB]

Sounds great. Photos?

 

[pro151]

It depends on if it's fine threads or course threads. It's been shown that course threads produce 20% more audio then fine threads. The course threads slow the RF down by reducing the velocity factor. Giving the radio more time to apply modulation before it radiates the signal, thereby increasing the audio duty cycle. Fine threads just act like a big loading coil offering tons of inductance to the skin effect unless coated with a generous amount of snake oil. OK, enough joking around, the threads do nothing detrimental to the antenna. I was just having fun. You've done yourself a fine job at building your own adjustable 1/4 wave antenna from available parts.

I had been thinking you were full of it and just trying to impress the "lesser" operators with your supposed "knowledge" of antenna and radio theory and now you just proved me correct!

If the threaded rod does not have left hand threads the reflect will be off the scale due to the gravitational drag coefficient on the radiated signal being applied at the base of the antenna. The only way to counteract this effect will be with a 10 turn balun at the mid-point of the antenna made from 500MCM cable

 

[HomerBB]

I had been thinking you were full of it and just trying to impress the "lesser" operators with your supposed "knowledge" of antenna and radio theory and now you just proved me correct!

If the threaded rod does not have left hand threads the reflect will be off the scale due to the gravitational drag coefficient on the radiated signal being applied at the base of the antenna. The only way to counteract this effect will be with a 10 turn balun at the mid-point of the antenna made from 500MCM cable

Have Mercy!, just when i thought I could make one of these antennas I see I can't afford one of these 10 turn balun at the mid-point of the antenna made from 500MCM cable :sad:

Seriously, BM, this seems a snazzy antenna. I sure would like to see it, though.

 

[loosecannon]

many times when we type LMFAO we arent really doing it, but Shockwave, that was F-ing huularious!!!

the best part was that YOU said it. i saw that you had replied to my thread and said, "cool, i wonder what he has to say about it." it actually took me up to the word "audio" to realize you were yanking my chain.
that was good.

homer, i know im bad about posting pictures of my builds, and whats worse is i always expect others to post pics of theirs. LOL i will try to get some pics of the beast.

i will be testing it this afternoon and will post the results this evening.
the wind doesnt seem too bad today, so im hopeful.
as long as it doesnt cave in the roof of my truck i should be fine.

last night my arms ached from threading that damn thing back and forth into the base. turning a 1 foot bolt by hand is not fun!
today i brought my 18v screw gun so that should help.

thanks for the input guys. ill be sure to provide data regarding the audio gain of this antenna.
im just not sure if audio gain is measured in PEP, RMS, or Db???
LC

 

[mackmobile43]

Makes sense to me, and I don't see why it wouldn't work just dandy. I have no idea how that threaded surface of the thing will affect 'skin-effect', but really doubt it it'll be a huge problem of some kind.
Corona ball? How about a couple of nuts to fit that all-thread, screw'em onto the top. 'Round' the edges off a bit if you want to.
- 'Doc

Just get a bullet head nut like the truckers use.

 

[loosecannon]

just thought of a few things.

first of all, Homer, who you callin' a BM?

i was also thinking it might be neat to cut different lengths of all thread for different bands, to avoid all that threading.

heck, it might even work on 6m!
LC

 

[BOOTY MONSTER]

It depends on if it's fine threads or course threads. It's been shown that course threads produce 20% more audio then fine threads. The course threads slow the RF down by reducing the velocity factor. Giving the radio more time to apply modulation before it radiates the signal, thereby increasing the audio duty cycle. Fine threads just act like a big loading coil offering tons of inductance to the skin effect unless coated with a generous amount of snake oil. OK, enough joking around, the threads do nothing detrimental to the antenna. I was just having fun. You've done yourself a fine job at building your own adjustable 1/4 wave antenna from available parts.

threads dont affect audio , they affect tone .
fine threads make audio with more treble , course threads make audio with more bass .

hey LC , if you have a circle saw get a black fiber $4 metal cutting blade for it . ive cut conduit and 1/8 inch aluminum plate , 3/8 inch solid aluminum rod and 1/2 inch bolts with it . being very careful.......the side can be used for smoothing and rounding edges and corners
BM

 

[loosecannon]

you betcha Booty. ive used those metal cutting circular saw blades to cut 3/16 steel plate before. they are loud and obnoxious, but they will work.
definitely want to be careful cleaning parts with the side of the blade. i have a friend with a really neat scar on his face from a chunk of a hot saw blade.
can be a great time saver, but watch your, well, booty.

cutting aluminum with them works fine, but im sure you've noticed that the aluminum actually melts to the edge of the blade. this can cause problems with cutting.
usually, aluminum is cut with a wood blade. just get one with a lot of teeth.
many people have given me a funny look when i tell them that, but in the metal shop, thats how its done. ive cut miles of it for making aluminum truss.


i got a quick minute to test the antenna and it seems to be working just dandy!
audio gain is off the hook!

i was able to get about a 1.3:1 SWR at the bottom of the curve, which sounds about right to me, with a 1/4 wave antenna having around a 36 ohm feedpoint impedance.

i actually cant plot a real curve with this antenna as i dont have enough freqs!
from 26.8 to 27.8mhz it stayed right around 1.3.

this was all measured with my DX400 inline and off. with the amp on, the SWR raised to about 1.7, which is a little odd, but doesnt really concern me.

the SWR stays pretty constant when using the amp with my wilson 5000 which gets mounted on the same mount.

go figure.

most importantly, it is a BEAST! this antenna will not be up on the roof of my truck in a high wind!
in this respect, the antenna is not the answer to all my problems like i thought it might be. it will be fun to play with and have, but i feel a revision is in my future.
maybe something besides conduit and use a set screw with an actual CB whip up top.

i will still try to come up with some pics, but it may take a little while.
i am always up for ideas, so feel free to suggest materials and such.
LC

 

[Shockwave]

Your best bet for cutting aluminum on a table saw (or similar rotary saw) is using a carbide tooth blade that you keep oiled between cuts. The carbide goes through aluminum like butter and does not melt to the blade because the teeth are wider then the blade and create much less friction. The blade should have at least 80 carbide teeth if you're cutting thick material. I've never tried it on anything harder then aluminum but it works great on that.