Antenna Feedpoint Coax Choke

[prcguy]

Finally got 12T of 75-ohm RG59 on FT240-61 done; pair. Oriented port & starboard. Was unable to assemble with split-loom cover in place.

Took pretty much the whole of 6’ jumper to accommodate bend-radius restriction (60mm). It also took time to figure out how to thread cable without inducing torsional twist.

Still need a thumb-size piece of wood dowel or pvc pipe to go in center: no cable overlap, and also against ring interior.

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Its predecessor was a tad bulky to fit and to be secured under antenna mount in use without constant revision. But proved concept over broad range of conditions.

— A “better” coax choice for 75-ohm (didn’t uncover that) which could be wrapped almost tight around ferrite toroid might use but 3.5’ of cable as reports of those using RG400 in 50-ohm apps suggest.

The addition of these jumpers causes the coax run to radio (each side) to now be 24’. Depending on which truck I’m in — if new coax run needed — I’m definitely going to cut down the harness leg lengths to what’s needed versus bundling up some of it.

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If all this is for a co-phased pair of antennas like on mirror mounts, you don't want to mess with the coax length of a factory co-phasing harness made of 75 ohm coax. The co-phasing harness is a 50 ohm power splitter and the coax lengths are critical for matching. If you need to extend a factory harness its best to add 50 ohm coax to the center common point or to extend the two antenna sides with equal lengths of 50 ohm coax since the harness is designed to provide a 50 ohm match where each antenna usually connects.

Once the co-phasing harness has done its job splitting the signal equally at the two antenna connections you can do anything you want on the antenna side as long as you treat it as a 50 ohm system from that point on. The reason you want equal lengths of coax on both sides if you extend them is different lengths will cause a phase imbalance between the two antennas possibly making them directional to one side or the other.

 

[merlin]

WOW. This thread boggles my mind. I started driving interstate semi back in 1972. The good old days of CB and big wheels rollin.
By that time, I was a decade amateur and ASEE. I specialized in antenna systems. This thread is so overkill people don't want to read through all of slowmovers specifics, much less understand. "High performance mobile CB" WUT? It doesn't work like that. 'DIPOLE' on a semi ? WHY,
you can get equal performance from 'ONE GOOD ANTENNA'. Co-phasing a pair of them will help radiation pattern where you want it, 'IF IT IS INSTALLED properly'. You don't have common mode problems if everything is grounded properly and that means no external chokes on coax.
Back on post #5, you don't radiate outside the coax or receive noise, with good coax.
You cant buy a truck stop radio and make it perform better than it does, not on 11 meters.
That is why I opted for a $1500 radio. Alternator whine, can't hide from that, but you can near eliminate it with an alternator noise supressor and maybe your stiff ferrite choke on the radio power leads. And keep leads as short as practical.
My Freighliner had a 100 amp service bus under the dash, even has a filter capacitor to ground. No problem there.
Running the I 5 corridor, Most drivers I could hear were out to 10 miles or more and If I could hear them, they could hear me.

 

[prcguy]

WOW. This thread boggles my mind. I started driving interstate semi back in 1972. The good old days of CB and big wheels rollin.
By that time, I was a decade amateur and ASEE. I specialized in antenna systems. This thread is so overkill people don't want to read through all of slowmovers specifics, much less understand. "High performance mobile CB" WUT? It doesn't work like that. 'DIPOLE' on a semi ? WHY,
you can get equal performance from 'ONE GOOD ANTENNA'. Co-phasing a pair of them will help radiation pattern where you want it, 'IF IT IS INSTALLED properly'. You don't have common mode problems if everything is grounded properly and that means no external chokes on coax.
Back on post #5, you don't radiate outside the coax or receive noise, with good coax.
You cant buy a truck stop radio and make it perform better than it does, not on 11 meters.
That is why I opted for a $1500 radio. Alternator whine, can't hide from that, but you can near eliminate it with an alternator noise supressor and maybe your stiff ferrite choke on the radio power leads. And keep leads as short as practical.
My Freighliner had a 100 amp service bus under the dash, even has a filter capacitor to ground. No problem there.
Running the I 5 corridor, Most drivers I could hear were out to 10 miles or more and If I could hear them, they could hear me.

The problem with some semi trucks is a large mirror on each door and nearly everything else is fiberglass. The mirror assembly is nowhere near large enough to provide a ground plane for the usually co-phased antennas on the mirrors so the coax becomes part of the counterpoise and its hot with RF. Adding a second tuned whip to the mirror pointed downward and grounded makes each mirror mount antenna into a dipole which is now mostly self contained and relies much less on the tiny useless ground plane its been given.

Adding CMCs to the feedpoint of each antenna minimizes RF on the coax and also helps keep RFI that might be picked up on the coax shield from reaching the antenna and being received to some extent. I first used the dipole concept on big rigs in the mid 70s working as a professional installer and it cured a lot of problems back then and still can today and 2ft or 3ft Firesticks were my antenna of choice back then for the grounded dipole side since we were a major Firestick dealer.

I had most of the big semi trucks mapped out in the 70s like which ones had more alternator whine, which ones had fiberglass doors vs metal and could make up a list of needed items for a potential customer before they even shut their rig down and walked into our store. There were a few steel roofed big rigs back then and my first choice for those was a good base loaded antenna in the center of the roof, even though it gets slightly blocked to the rear from a tall trailer.

 

[merlin]

To a level, I agree. Newer mirrors even lack upper and lower brackets, make mounting an antenna impossible.
But still, mirrors are not mounted to fiberglass, there is door frame underneath, and then a good ground strap frame to door frame.
IF there is no other option but mirror mounting, then the dipole should work fine, but I would mount to the cab before that option.
Simpler is better. My truck, the left mirror supported a Hustler RM-11S on a 15 inch rigid mast. The right, the same thing except a
RM-40S . That one just would clear 13'6".
I did have option to mount a Wilson 1000M center of the cab. That part aluminum and long sleeper would have worked, but the 1000M
I used on my pickup with mag mount.