The 5/8-Wavelength Antenna Mystique by Donald K. Reynolds

[BOOTY MONSTER]

hi guys. interesting topic for sure. ive always wondered if the imax 2000 and a99 antennas were the wire lenghts just happen to fall at the right wave lenght size everytime ? how could this be possible? is there something iam missing? because surley you would have to take into account the vf of the wire used AND it being encapsulated!!!! yet it still has the right lenghts for said antennas! ie 18 and 24 foot !!! 73s.

The Imax 2000 EXPOSED!

" ...... The first thing you notice about the Imax 2000 is that it is LONG! I measured the copper wire elements after they were removed from their fiberglass radomes:

Bottom section: 80 inches
Middle section: 94 inches
Top section: 96.5 inches

That makes the total radiating element length 270.5 inches. Using 27 MHz. (CB) as center frequency (which this antenna was designed for), that makes the Imax 2000 a 0.640 wavelength antenna. ..... "

"...... In photo 2, you see what is inside the lower section and how the tuning rings affect the coil's tuning. Except for the lower 2 feet of the Imax 2000, all the rest of the antenna contains nothing more than a straight piece of #14 bare copper wire. ......"

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just thinking out loud .........
23x 12 is 276 inches , add a few inches for each of the connections on each end of the wire and they probably add another 4-6 inches . so if there is a VF caused by the radomes (FWIU there is) then the antenna may be longer than .64

make any sense ..... ? or am i still the rightful possessor of the dunce hat

 

[Marconi]

The Imax 2000 EXPOSED!

" ...... The first thing you notice about the Imax 2000 is that it is LONG! I measured the copper wire elements after they were removed from their fiberglass radomes:

Bottom section: 80 inches
Middle section: 94 inches
Top section: 96.5 inches

That makes the total radiating element length 270.5 inches. Using 27 MHz. (CB) as center frequency (which this antenna was designed for), that makes the Imax 2000 a 0.640 wavelength antenna. ..... "

"...... In photo 2, you see what is inside the lower section and how the tuning rings affect the coil's tuning. Except for the lower 2 feet of the Imax 2000, all the rest of the antenna contains nothing more than a straight piece of #14 bare copper wire. ......"

-----------------

just thinking out loud .........
23x 12 is 276 inches , add a few inches for each of the connections on each end of the wire and they probably add another 4-6 inches . so if there is a VF caused by the radomes (FWIU there is) then the antenna may be longer than .64

make any sense ..... ? or am i still the rightful possessor of the dunce hat

For me BM, the lower part of the antenna inside of the mounting tube probably doesn't radiate very much, and I doubt much of the tuning device above the tubing...adds much either. Other than maybe the bronze capacitor...the 270.5" wire length that Tech 833 writes about, is probably all that actually radiates.

I don't see the wire connections you note, but I know there are two ends inside of the capacitor. Each wire is mounted in Teflon plugs and these plugs provide a small air gap and they don't connect creating a capacitance.

So no, :thumbdown: this idea doesn't make any sense to me.

I also don't believe the Imax is anything but an effective .625 wavelength radiator.

When you built your 5/8 wave, did you include the length of the Maco type coil or the length of the smaller air wound type coil in figuring the overall length of your radiator, thus making the radiator shorter in the process? Or did you just get the radiator length close enough using your formula, and then hope the coil would tune where you wished?

BTW, I don't recall you posting much about your 5/8 wave before you started with your Sigma styled antenna, so how did the length and tune of your 5/8 wave work out in this regard? What were the numbers and formula you used?

 

[Needle Bender]

Oh I was kinda sleepy when I responded, but when I did his math, 599/freq = about .608 or about 5% shorter than the .64 he mentioned, which would tune a 27.2 dipole to about 28.6, not an insignificant amount of error.

 

[BOOTY MONSTER]

i just measured the vertical at 22 1/2 ft and 23 ft , from where the ring attached to it , not including any of the loop/coil for matching . at that location the only other antenna i had used was the wire starduster i made . so it's not a fair comparison IMO . but with the SD's feed-point at 17 ft and the 5/8 or .64 20 ft across the yard and at a feed-point of about 8 feet the 5/8 did much better . it got even better when i got it up to a 16 ft feed point .

but there could have been something wrong with my SD's build . you mentioned the SD's patent over at the quack shack and my version had 3 obvious differences to me , maybe more to you or someone else . according to the patent my ground radial angle was too wide , i didn't have a conductive mast at the feed-point running down the center of the ground elements , and my coax wasn't running up the center of the conductive mast .

the SD did have a donut shaped area about 20-25 miles out where it just didn't want to hear well and didn't tx real well either even with my little 2 pill or the 500v on . at about 30ish miles it did well again though . the 5/8 at 8 ft didn't have any hint of a dead zone and of course was mo' better up above the roof .

 

[Needle Bender]

I wanted to find a nice and easy formula to find the wavelength in air if I entered the frequency in MHz so I did the math:
light = 30000000000cps / 2.54cpi = 11811 - the number I was looking for.
So,
11811/27.185 (center of band) = 434.5"
434.5 x .64 = 278"
434.5 x .625 = 271.5"
If the Imax comes in at 270.5" then it is 1 inch short for a .625 (5/8) which I bet is made up for by the fiberglass medium slowing the velocity factor a bit.
I believe the imax is a 5/8 not a .64

Another anecdotal comparison was done last weekend. I helped erect a Sirio Gainmaster in place of a good working Imax on top of a 36' telescoping mast from the ground up, in a backyard, 80' from the house, and using dacron guy line, no metal.
Coax was LMR-400.

The Gainmaster was actually lower in transmit signal strength to a couple stations about 35 miles away.
I wonder if the lower angle of the Gainmaster didn't get his signal up and over local hills as well as did the Imax with it's higher toa.

Maybe the Gainmaster doesn't like being right at one wavelength above ground

Basically they performed almost exactly the same nearby, a few stations in the distance saw improvement and a few others in a different direction saw a decline.

Overall his receive is improved but only a little bit and not in all directions.

Surprisingly, for his application, the Imax might be the better performer.