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Firestik "Tip Cover" Doubles my SWR?

10K views 25 replies 12 participants last post by  Mr. Bills 
#1 ·
I am in the midst of my CB setup. I found a good mounting location (back left of roof) and believe I have all the necessary components. I wired everything up and went to test out my SWR using some $20 reader I got off Amazon.

Using channel 19, I was able to tune my 3' Firestik to 1.3 SWR (plus or minus .05). At channel 40 it was about 1.5, and at Channel 1 it was about 1.6 -1.7. good enough, I thought.

Then, I put on that rubber cover over that tuner on the antenna, and check my SWR again. It has doubled to over 3.0!! Why? Does it effectively change the length of the antenna? Am I supposed to just leave it off?

I'm talking about this thing:



Here are some pics of my setup for reference:




 
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#2 · (Edited)
More setup details:

  1. 3' Firestik II with Heavy-Duty Spring
  2. Midland CB Radio
  3. 18' Coax Cable
  4. 3/8" (or 1/2", maybe) braided copper wire grounds it to the roof

For testing the coax cable was just running alongside the outside of the truck as you can see in the pic above. It wasn't coiled or anything.

Also, for the braided-copper antenna ground, I checked continuity using a voltmeter between the threads of that roof rack screw and the threads of another screw across the roof. It was solid so I grounded it there (does it need to go to frame?). I think it OK because I was able to get 1.3 SWR.

My radio was plugged into the DC outlet in my center-console.

For SWR test I was right next to my garage with a fat metal roof. Could that have made the SWR more susceptible to change?
 
#3 · (Edited)
Some initial thoughts:

1. When you tune your antenna, do it out in a field or someplace as far away as possible from buildings, trees, etc. It does make a difference.

2. After you adjust your antenna, replace the plastic tip before you measure SWR. In other words, take your readings with the tip in place. A bit of WD40 helps in getting the tip on an off. On edit: Run your coax as close as possible to the way you will run it on a daily basis when you check your SWR. Readings will change if you re-route your coax after tuning

3. I apologize for being blunt, but your antenna mount ground strap is useless. You need to have a solid connection to the body. The roof rack bolt you chose threads into a non-metallic riv-nut filled with thread lock, i.e. no direct connection to the body. You may have electrical continuity, but you have not bonded your antenna mount to the body sufficiently to create a RF ground plane. That is why I chose to drill a hole and mount my ground strap under the hatch, shown here:





CB antenna tuning instructions with video:
CB Antenna Tuning Instructions | Right Channel Radios
 
#4 · (Edited)
My 2' fire stick did the same thing. I couldn't get it to tune to less than 2.0 swr on channel 1 or 40 with the cover. 1.3 on both without the cover. I tossed my cover in the trash.
 
#6 ·
I was thinking this because I don't see a problem with it... maybe the tuner gets messed up if it hits a tree or something, but other than that I don't see how that could hurt.

Another thought, when you apply the Cap. Are you rotating cap as you install it, which is moving the Tuning stud?.

Your grounding stud.. seems weak.. might be the culprit.
Grounding stud, might need more Ground connection. Add Jumper cables to test, for the grounding.
I was just pressing the cover straight down. But good idea with the jumper cables...

3. I apologize for being blunt, but your antenna mount ground strap is useless. You need to have a solid connection to the body. The roof rack bolt you chose threads into a non-metallic riv-nut, i.e. no direct connection to the body. You may have electrical continuity, but you have not bonded your antenna mount to the body sufficiently to create a RF ground plane. That is why I chose to drill a hole and mount my ground strap under the hatch, shown here:
Haha! I appreciate the bluntness. Even after a lot of reading this was the thing I was still most confused about (what works as an antenna ground and what doesn't). I figured since my SWR was good that it was working, but I guess not.

I'll fix my ground, move away from the garage, and see if I have the same problem. Thanks for help.
 
#5 ·
It would not hurt to try moving away from the Metal Roof. try open spot, for better Radiation pattern of your antenna.

Another thought, when you apply the Cap. Are you rotating cap as you install it, which is moving the Tuning stud?.

Your grounding stud.. seems weak.. might be the culprit.
Grounding stud, might need more Ground connection. Add Jumper cables to test, for the grounding.

Just few thoughts. I ditched CB's and switched to the dark Side of Ham.
 
#10 · (Edited)
I didn't like my firestik much, but that was on my Frontier...going back to a whip antenna when I do the X (hopefully soon!)

Anyway, IIRC, there is some wire that can and needs to be trimmed for SWR adjustment, amongst other things (way too many to list). That is for fiberglass types, like the Firestik; for whip/ss types, you trim off the end of the 'rod', accordingly. BE ADVISED though to do it slowly, as once you go too far with trimming, it's really difficult to go back.

If the cap is affecting SWR, then thinking perhaps it's moving that wire underneath it inadvertently or something.

There are some good suggestions in here, especially with regard to the ground wire not being effective in this case, and to tune as far out in the open as possible, etc.

EDIT - from Firestik's instructions: http://www.firestik.com/Tech_Docs/Setting_SWR.htm

Note the warning about the tip.
 
#12 · (Edited)
I found that mounting a 4' FireStik II using the FireStik door jamb mount (requires just a little bit of bending the bracket with a heavy vise) behind the passenger tail light (use the 4 included SS screws directly into the metal). This particular antenna configuration resulted in a 1.00:1 across all 40 channels on a Uniden Bearcat 980SSB. I have done similar setups on 2 other X's as well and everyone was 1.00:1 using the built in SWR meter and < 1.2:1 when checked with a Comet CAA-500 MkII antenna analyzer. The antenna where the majority of the coil resides is above the roof line which results in a fairly good radiation pattern. Coverage is excellent, and SSB works great as well. I tried installing a HAM antenna on the same mount, but the SWRs were terrible, for HAM I used a lip mount on the hatch as close to the top as possible, this resulted in great SWR <1.5 for 10/6/2/70cm. Out of curiosity, I switched the antennas around and the SWRs for the exchanged antennas were absolutely terrible, > 4.0:1 using a Comet CAA-500 MkII and upwards of 6.0:1 where the analyzer gives up.

My friend that has the same exact radio/antenna configuration as me, with same SWR readings, but managed to lose the red FireStik II tip due to driving in an underground parking garage, SWR shot up to 3.3:1 no matter how we tuned the antenna. I removed the cover from my antenna as well and noticed the SWRs shoot up like crazy, I ended up with > 3.0:1, drop the cap back on and back to 1.00:1 across the whole 40 channel range. It sounds like you may have grounding issues/insufficient ground plane or your coax isn't cut to the proper length. This is far more noticeable at higher frequencies, but it may help to experiment with your coax length, trim a little at a time and retest. I find that trimming the coax to get optimum SWR for the 440 MHz band generally yields great results for HF/VHF frequencies as well. I notice for HAM antennas in the UHF range, you get some really interesting looking frequency filtering between the 420 MHz and 450 MHz range if the coax is not trimmed properly.

Edit: As you may be able to tell from my avatar, I ocassionally run a 102" SS whip, I found that this works pretty well for 2m (HAM) as well as 11m (CB) operation. I use the SS whip where there may be overhead obstacles as it yields more. Same idea with the coax though, it needs to be trimmed to the right length for optimum SWR. Ed Fong once told me "You can trim the antenna to tune to your coax, or you can trim your coax to tune to the antenna." I chose the latter. The DBJ-1 antenna great for stationary set ups, compact and unobtrusive j-pole for 2m/70cm operation.
 
#14 · (Edited)
Firestik &quot;Tip Cover&quot; Doubles my SWR?

Stay away from anything metallic when tuning swr.
Even opening a side door or hood will play havoc with reflectivity if near enough.

Don't forget that anything else you add to the antenna such as spring, fold over mount, quick disconnect mount, etc will affect tuning as it is part of the antenna.
 
#16 ·
Let's see some pics of how you are mounting/grounding it. There are many factors that could affect, but antenna location & ground I believe are the most important
 
#17 ·
I use this mount on my hatchback with my Firestik: Diamond® Antenna ~ K400 Trunk/Hatchback Mount. I don't use a ground strap. I tuned it with the cap on, so you take the cap off and put it back on with each adjustment. SWR is 1.2 tuned on channel 15. On channel 40 it was about 1.8 if i remember correctly. It is a fairly expensive mount (~$70) but the cable and mount quality make all the difference.

 
#19 ·
The primary issues I see (and can't see) with your antenna system are:

1. Grounding - be sure that you have a good ground between the mount and the body where it is attached with the 4 screws. The screws appear to be painted or coated. If the threads aren't bare metal then you have no ground at all. You may want to use zinc plated or stainless steel screws instead. Also, be sure that you have a good metal to metal connection where the two pieces of the bracket attach.

2. Location - The optimum antenna location is in the middle of the roof. Mounting the antenna low on the body impacts SWR. However, it appears that you still have at least 2 1/2' of your toploaded 4' Firestik antenna above the roofline so you should be able to get acceptable SWR readings.

3. Stud Mount - I cannot tell how you assembled your stud mount because it isn't pictured. A washer in the wrong location will result in extremely high SWR readings. Be sure it is like this:



4. Coax Cable - I can't see this either. Be sure it doesn't have any sharp bends or coils (excess cable should be in skein or figure 8).
 
#20 ·
Those screws came with the mount and are metal. They are also into unpainted holes in the body. Not sure if there is a good way to test for ground otherwise.

I'll have to check the stud mount. I dont have a pic of it but I thought I had it right.

The cable runs under the interior plastic paneling to the front center console. There isnt much extra cable but I'll double check that too.
 
#22 ·
Went out and checked the stud mount and it is correct.

The screws appear to be just polished metal. I dont see anything that looks like coating on them.

I used my multimeter on the 200 Ohms setting and get .5 to .1 when I touch one side on the mount and another on a body location (found a hole in the tail lite assy that doesnt look like its painted inside). Not sure where else to try for ground test back there, not many choices that I can see.

Also, not sure how much it matters, but my radio plugs into the 12v outlet which may also provide a ground.
 
#26 ·
You are still confusing electrical ground with ground plane. You don't need to run a ground wire to the engine bay for either the electrical ground for your power circuit or to improve ground plane for your antenna.

For your Midland 75-822 - the only way you can power it is to use your 12v socket, which is already sufficiently grounded. No need to add a ground wire.

For your antenna - you want to electrically bond your antenna mount to the body. If you add a ground strap or wire for this purpose you want it to be as short as possible.
 
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