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Discussion in 'CB Antennas' started by ctvanover, Jan 3, 2011.
You do not have to use 18', keep both sides even to a T connector with 75ohm coax and then 50 ohm coax to the radio or a 75 ohm co-phase harness that reaches the antenna it will make it's 50 ohm connection at the radio, short is more gooder.
I use to help build tuned co-phase harnesses years ago. We didn't understand much about VF and specs on cable back then was dubious. So, we tuned the harnesses to work at 27.205 for CB and recommend their use on P/U trucks rear bumpers with 102" whips. Tuning the harness accounted for the variability of VF in the cable used.
This tuning process took care of whatever the VF was for the coax we used. In tuning both RG6 & RG59 we noticed at times the tuned length of the harnesses did change a little, so we figured the cable's VF was the difference.
Each harness was hand made and tuned. We did not build these in the traditional 18' foot length, which is electrically about the same with 3 times the side length of our shorter custom harnesses.
We started with a length of cable about 13' feet long, doubled it over and carefully cut a 2" space in the middle of the coax to expose the shield. We then carefully spread the shield apart and fished-out the center conductor with all wire left hole and intact, with no nicks either. We then carefully removed and inch of insulator without affecting the center wire and created a short twisted pig-tail for each conductor, being careful to keep both uncut with no nicks. This pig tail was installed within a coax connector and that would accommodate a standard barrel connector and soldered well. This end was later connected to the random length working feed line to the radio.
The open ends of this harness were also stubed-out and attached to the antennas and tested for resonance. These ends were made a bit long and then trimmed down to center frequency for CB. Each end ended up being about 6' feet long per side and would easily reach across the back end of a P/U truck.
If you want to get closer to start you can use the following formula.
492 x VF / frequency = total harness length.
492 x .66 / 27.205 = 11.94' which is then folded over.
11.94' / 2 = 5.97' x 3 = 17.9' this is where the 18' harness comes from and is electrically similar to the shorter harness.
Again to be safe make your cable a little long in case the VF for the cable used is not what is published. Then fold this in half and follow the instructions noted above.
We also water proofed all cable ends. You will note when finished that you will have 2 x 1/4 wave tuned wires that help to transform the low impedance match for each whip upward closer to 50 ohms when combined in parallel. The best place to test the SWR before installing is right at the pig tail end. Make sure the truck is in the clear for testing.
I make no claim for this working as intended on the big truck, but your phasing harness does not need to be 18' feet long, unless you need the added length to reach.
We found that Field Strength was improved noticeable using the shorter harness vs. the 3x cable when tuned properly for the antennas. This is due to the increased cable losses in transformation. You may also find this setup to be rather narrow banded.
With a big truck one antenna may actually work better and possibly provide added bandwidth to accomadate modern broad-band radios. Keep me posted on your progress.
That's an excellent formula Marconi, in the semi-truck installation there will need to be a bit more length for routing of the 75 ohm harness so it may be necessary for ctvanover to use the formula (11.94' / 2 = 5.97' x 3 = 17.9') to achieve the length required for his install which would make either side 9' so if he can find a 75 ohm harness that has 2/9' sides then he's set or by making up 2/9' 75 ohm length coax leads and join those with a T connector then a mini 8 lead from there to the radio.
Now, how would you go about making the phasing harness that would change the radiation direction 90 degrees? Make it just a tad easier/harder and only mess with changing one side of that harness so you can make the whole mess 'switchable'!
Doc you sure stir it up
Force current feed it with a switchable coax harness cut to the degree of phase you want to shift. It then can be switchable to be and end fire array in two directions or a co phased.
ON4UN low band dx book has a lot of information on this type of antenna. I built one for 40 meters, worked ok for a ground mounted vertical array.
Yep, when I wrote my part of that article, I had to sign a none-divulge contract...not to ever tell more details. If I told you more they would hunt you down like a dog :sad:. Our authority for production came from the International Atomic Energy Commission. IAEC., a divsion of EI-EIO, LSMFT.
Except for what is currently published, we are still held to top secret security clearances. To even speculate can be dangerous to life and limb.
I understand that EI-EIO was bought up by McDonalds a few years back.
Lucky Strikes--- I dunno
ctvanover, don't let 'Doc corn'fuse you. He's right, this will work if your antennas are matched very close, with matching resistance near 35-39 ohms as installed individually.
Like I said earlier, we found that actual field strength fell off rapidly if more tuned coax was added to the 1/4 wave transformers in order to reach.
The transformational losses appear to be squared in magnitude with each additional 1/4 wave segment, and if the initial input self-impedance for each antenna is lower than about 33 ohms at the feed point...the losses may null the system...even if the SWR match looks good. Such detuning typically occurs when a suitable ground is insufficient to produce 38-39 ohms of resistance at the feed point for 102" whips. This setup will not be very broadbanded, but the 102" whip shows a very good bandwidth...well able to cover the entire 11 meter band at low SWR. Adding extensions for resonance is not necessary, and is not encouraged for rear bumper P/U with antenna stud mounts only.
We also suspected that these co-phase harness setups seem to work better when mounted closer to the ground, albeit ground losses do increase. This is why we recommended installation on rear P/U bumpers using 102" ss whips only.
If any of you guys try this scheme, post and let us know how if goes.
RG-12? Don't believe I've ever heard of it.
Now, RG-11 is good coax, 75 ohm, same dimensions as RG-8 or 213....
Congratulations! YOU just found ARRL re-inventing the wheel.
All that info can be found in Terman Radio Engineering 1943 and was well known way before that.