Tuning a Francis antenna?

So I came to this thread because I found it a while back.

Just got in a new truck with my company after several months of recovering from a broken arm.

One of the stock mirrormount antenna's on the Prostar was broken off so I held off on hooking up my CB for a couple weeks because I knew what the outcome wpuld be (I'm new, not stupid). One day I was at a truck stop and they had the Francis Hotrod 400 (4') on sale 2/$32 so I got a couple. Replaced the... Crap what are they called... What the antenna screws into on one side and the cable on the other side that fits in the mount? Anyway, replaced both on both sides so that they would be identical. Even there the mount appeared to be properly grounded (its a 2015. I know the 2010s thru 2013s did NOT have a good ground because even through the mirror bracket was metal the mount it connected to was plastic) I still took some outdoor 10Ga (AWG 20 I think?) wire and rant it from the bolt that holds the bracket to the mount and the screw that holds the mount to the body on each side just to be sure I had a good ground.
In between runs the guy at the shop was like "hey, wasn't one of your antenna's broken? We finally got more in from the parts guy because there were 4 or 5 tractors with broken ones and they have to be fixed" so on my next run I was out in Bumfolk Indiana in a bunch of flat empty fields, found a place I could safely pull over where there were no powerlines, signs, NOTHING. So optimal spot to check SWR. Also had a trailer on the tractor so I figured it would be better for a solid result of what I would be looking at.

Anyway, pulled over, hooked up my CB (hadn't even plugged it in till this point) and checked SWR with a free standing meter with the stock antenna's. I figured maybe the factory #### would be tuned specifically for the truck.
NOPE!
SWR was like 2.8 on ch1 and HOLY WHAT THE HECK! On ch40.
I didnt expect much better from the Francis 4' considering they were the same length only about half the diameter. But the Francis' 4s came out to 2.1 of ch1 and 2.3 on ch40. With a trailer connected... I can live with that.

But Ive been doing a lot of thinking a research. With it being a sunrise cab I can't break the verticle plane of the roof and cab. Being mirror mounted the maximum length I can go is 5' (60"), and with the trucks being leased by the company I cant make any permanent changes of adjustments that would be noticeable. Almost nobody make a 60" except ProComm and Ive never even heard of them before. Predator makes some good ones, But they're 63".

So it looks like my best bet is the Francis 4.5. They're a double quarter-wave (so 1/2) constant double helical wound from what I understand. So youre not actually supposed to cut them because you can muck it up big time.

My question is can it be done with cable calculation?
The antenna accounts for 1/2 wave so the cable would have to account for 1/2.

How would one "tune" an antenna such as a Francis 450 (4.5')? Does one need to tune an antenna if they properly calculate cable length? A Francis 4.5 is a double quarter helical wound "preset/tune-free" antenna. So it's 1/2 wave. So say I was using Belden 9259 which has a velocity propagation of 78%, my target Freq was 27.185 (channel 19); 984×.5=492×.78=383.76÷27.185=14.11ft? Would this work?

Would this be a possibility or no? And is my math correct?
Because I CAN change out the cable without it being noticed (I wish I could find a video on how to make cophased cable ends. There's a million how-tos and videos for regular PL259 ends but not for cophased.) And the company already knows I put the Francis on there, they said as long as the shop gets the originals back they dont care.

I would like something with the best TX/RX I can conceivably or possibly get without burning out my squeak box. And I want to heard and be heard clearly. I like to get at least a few miles of a heads up on some BS if I can.
And are firestick genuinely worth it or is it a lot of hype as I suspect?

Any advice would be greatly appreciated. Thank you.

 

Although some sources may disagree, my observations are that it's not usually recommended to tune the antenna via coax length. Generally, the amount of coax needed to span the antenna - radio distance is the recommended length to use. Trying to achieve better SWRs via additional coax length may work to some degree, but the only result is a lower SWR without commensurate performance enhancements.

 

Unfortunately I’m not familiar with the Francis hot rod double quarter wave mobile antennas. So I can’t be much help there. You are correct with your math as far as coax length and velocity factor is concerned. There is a way to make the math simpler, and that’s to drop the full wavelength number (984). Since most antennas are based off of 1/2 wavelength, simply use 492*.78(vf)=383.76 / 27.205 MHz = 14.1 ft. Anytime the coax length makes a difference in swr measurement, it’s usually an rf ground problem like poor ground plane.

 

Night Stalker. Thank you for your input.
So youre saying donçt try to achieve a full wavelength with coax for a lower and more accurate SWR but rather go point A-B?

The reason I go with the full wave calculation is the other possible option would be a Firestick II 5' which is a 5/8 wave and has a tunable tip. The math comes out to like 10' 7.586" so 7.6 SHOULD make it slightly too short so you can tune out. I've found it's easier to add length with washers or a bltunable tip or whatnot than to reduce length. Also I'd rather add a couple washers or whatnot than to cut down and antenna because once you remove something it's gone.

The ground plane on the international I know is solid.

I understand perfectly the concept of "Length=Loss" but I also understand the concwpt of incomplete waves or oscillations equals noise or a sactifice in clarity. If you look at the numbers... Like most of the RG 59s have around 10dB/100' as 900MHz or something like that. For this specific one the dB loss per 100ft should be around 1.5-1.8 at 27MHz... And most average CB antennas will need LESS than 18' to tune properly. I dont understand all the other numbers yet. I'm still trying to figure all this out. In not worried about looking a fraction of a watt or 1dB of volume if it reduces noise, increases clarity, reduces SWR and and helps make things easier on the instrument. Which an RF receiver seems to be a pretty delicate and tickly instrument. Just like any instrument if you play it wrong you get sour notes and a headache.

My goal is to NOT be that big loud idiot that talks over everybody, has the echo cranked all the way, amp cranked all the way, modulation cranked all the way, dynamike, etc and you still cants hear what he saying whether youre right next to him or 5 miles down the road where he's slightly less loud, or 15 miles down the road where he's still loud, but its all echo and static and you STILL cant understand more than maybe a word or 2.

I wanna be that guy that can hear or be heard from 6 or 7 miles down the road (maybe if lucky) but whether he's right next to you or 15 miles down the road you can either hear him more clearly than if he was shotgun in cab.

Also trying to learn how to mod in suck a manner to increase power output, modulation, distance etc while still having almost unmatched clarity. And zero bleed/splatter/hemorrhaging.lol Idk if its possible. But it would be awesome. If I'm cussing someone out on 19 (27.185) is dont want the little kids in Chick-fill-a to hear me coming through on 27.187.

But so far I can only find where people want to make nouse blankets gates or filters for stopping incoming bleed/splatter rather than outgoing.

Sorry for babbling. I was just curious about the length question. If its possible to get a better SWR but cutting new cable to match the antenna instead of trying to cut the antenna to match the cable.

Also that Belden 9259 seems appealing but ot has a foamed polyethylene conductor insulation, sounds like the kind,of thing where having to serpentine that extra length would eventually allow that center conductor to cumpress or cut through the insulator and short out with the outer shielding. I remember a forum where someone explained the perfect insulator type for mobile appliactions to address such an issue. But I cant remember what the material was. I think it could only be found in RG-8A/U though.

 

I wonder if I could CREATE a helical wound or coiled 3/4 or 7/8 wave antenna so that only 3.5-7' of cable is needed.

 

Further research reveals anything above 5/8wave is garbage because it puts almost everything up vertically into the air and almost nothing horizontally along the horizon where 5/8 are almost ideal/optimum for horizontal transmit and recieve.

 

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.For the record, when you have a SINGLE ANTENNA SETUP you need 50 ohm coaxial cable. When using DUAL ANTENNA SETUP you must employ a co-phased antenna wiring system.
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.I believe you are way over thinking this coax & antenna thing. If you want a longer antenna than the 4 foot Francis there are other options. Yes, the 4.5 foot Francis is one option. The Firestick FS5 5 foot antenna in either the cut-to-tune variation or the 5 foot adjustable tip to tune antenna would work. So would a Wilson 5 foot Silverload antenna. There are others,I assume, that you could find. .
. When it comes to coax, you do not tune the coax length but rather tune the antenna. Changing your coax length may fool you meter but that is defeating the purpose of tuning your setup.
The longer your coax the more signal loss you incur. Shorter is better.
. You can spend alot of money on equipment for minimal improvements BUT personally I would not waste my time or money worrying about halfwaves or anything else about either antennas or coax.
It is CB radio from way back when. If you are dedicated to really long range communications try microwaves or the old standby....an amplifier.

 

Are you sure about that?
Yes, all my Coax Cables are 75 Ohms

You can use any kind of coaxial cable you want and it doesn't matter what impedance the cable is if you know what you're doing.

 

Sorry for the late reply. As far as coax length goes, its really a personal choice. What I mean is, it depends on if you want every ounce of signal to get out of your antenna, verses just being happy using your cb radio for short range communication that it was designed for. In other words there are guys who want all the watts the radio can make, to get out the end of the antenna, which is a good thing. So, in that case, you have to figure in the Velocity Factor (the speed that the electromagnetic wave travels through the coax). As you know, the Voltage is 180 degrees from the Current flowing through the coax, you want to cut your coax where the Voltage and Current meet. Now some say you want to cut your coax in multiple of 1/2 wavelengths only. So when I decided to replace my coax last year on my base station, I used RG213. Since this coax has a velocity factor of 66%, a 1/2 wavelength is 11.9ft. Well the problem is, my antenna is 63 ft from my radio. So according to this rule, I had to cut my coax every 1/2 wavelength instead of at a full wavelength. So when I took my 1/2 wavelength of 11.9ft and kept adding this length to itself, I came up with these numbers. 11.9, "23.8", 35.7, "47.6", 59.5, "71.4", & 83.3ft.
Now the numbers in perenthesis are full wavelength. Well at 59.5 ft, my coax was too short to reach my antenna. Since the next 1/2 wavelength is 83.3ft, that is what I should have used. The problem is, I didn't want to buy all that extra coax just to go 63 ft to my antenna. So I chose the "just to go from the radio to the antenna" option. I've always been the person who "used just enough coax to reach" type of person. Now in my mobile installation, since the RG8x coax I used is 76% VF, I did cut the coax to 13.7 ft, and the antenna did seem to tune easier. As far as dielectric material goes, Polyethylene "PE" is a little better than the foam Polyethylene. I did some research on the Francis hotrod antenna, it looks like the manufacturer just simply paralleled the helix wire. Not sure if its to mimick the folded dipole idea or not. As far as tuning the antenna, I see that CB World says to just simply cut the top off with a hacksaw to shorten the antenna. On the last note, since we are talking mobile installation, don't worry too much about the DB per 100ft loss in the coax. Since most mobile coax installation is less than 20 ft, you won't notice a difference in performance as far as loss goes on the 27mhz band.

 

In my CB installations, all mobile, I never worry about the coax length, velocity, etc.

KISS: 50 ohm coax long enough to connect radio-to-antenna, trim the antenna to below 1.5 SWR, done. Works good.