Question about mobile whip, first I know by having it where it is located is not optimal. My question is why did I have to add so much to this 102” whip to get it resent and maximize my swr. Overall length is 114” on 27.205
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11 meter whip length
- Thread starter Alabama Flash
- Start date
Welcome to the Forum!
Minimize the SWR is more appropriate…
The last time I analyzed a 102 whip I saw a reading similar to what you got but at a frequency around 27.750 MHz. Adding 2 springs will increase the length and lower the SWR.
If you weren't aware previously, Your most accurate antenna measurement for analysing is to use a short jumper from the analyzer to the antenna. Make sure you are doing this before making changes.
Your SWR with one spring on the antenna should be good enough. The difference between a 1.1 and 1.3 SWR is a 4% radiated loss. This means a loss of 1/4 watt on 10 watts.
I wouldn't get to hung up on getting a flat match. A 1/4 wave whip exceeds loaded whip antennas on E.R.P. due to losses in the loading coil. Your good to go with what you have with 1 spring or try to find a 108 inch whip.
Minimize the SWR is more appropriate…
The last time I analyzed a 102 whip I saw a reading similar to what you got but at a frequency around 27.750 MHz. Adding 2 springs will increase the length and lower the SWR.
If you weren't aware previously, Your most accurate antenna measurement for analysing is to use a short jumper from the analyzer to the antenna. Make sure you are doing this before making changes.
Your SWR with one spring on the antenna should be good enough. The difference between a 1.1 and 1.3 SWR is a 4% radiated loss. This means a loss of 1/4 watt on 10 watts.
I wouldn't get to hung up on getting a flat match. A 1/4 wave whip exceeds loaded whip antennas on E.R.P. due to losses in the loading coil. Your good to go with what you have with 1 spring or try to find a 108 inch whip.
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To answer your question better, on a mono band whip there is only one sweet spot. As you increase or decrease the antenna length you start moving away from that spot.
If you decrease the length too much, then you'll need to add a loading coil to bring up the impedance to get a good match. if it's too long, then you need to trim it down.
This is where you can see the bandwidth of the antenna. That refers to the useable range above and below the sweetspot typically at 1.9 :1 on both the high and low side and the lowest SWR in the center. You can operate your radio anywhere along this range and still be effective. But there is still only 1 sweetspot and it might be only a few KHz wide.
Your analyzer can scan and show this. That's what makes the Rigexpert analyzers ideal. Set the bandwidth at 1000 KHz and set the frequency at the center where you operate and scan the SWR bandwidth using keypad 4.
After scanning, You can move the cursor over on each side towards 1.9 SWR and note the frequencies. That's your operating range.
Hope that all helps!
If you decrease the length too much, then you'll need to add a loading coil to bring up the impedance to get a good match. if it's too long, then you need to trim it down.
This is where you can see the bandwidth of the antenna. That refers to the useable range above and below the sweetspot typically at 1.9 :1 on both the high and low side and the lowest SWR in the center. You can operate your radio anywhere along this range and still be effective. But there is still only 1 sweetspot and it might be only a few KHz wide.
Your analyzer can scan and show this. That's what makes the Rigexpert analyzers ideal. Set the bandwidth at 1000 KHz and set the frequency at the center where you operate and scan the SWR bandwidth using keypad 4.
After scanning, You can move the cursor over on each side towards 1.9 SWR and note the frequencies. That's your operating range.
Hope that all helps!
Correct me if I am wrong, but it looks like the antenna is bolted to the side of the toolbox? If that is the case make sure that toolbox has several good DC contacts to the bed of the truck on both sides of the toolbox. When it comes to this, more is better. The toolbox, in and of itself, is not helping the situation. If you don't have enough electrical connections to the bed of the truck, the toolbox will try and be all of that antenna's ground plane, and as it is a bit small to be a complete ground plane, you would compensate by making the antenna longer.
Being that you know this isn't an optimal location, that is also part of why you have to make the antenna so long to get to the resonant point as well as based on the readings of the RigExpert, the low SWR point. Antennas like to be in the middle of their ground plane, Moving the antenna away from said optimal location, you need to make the antenna longer to compensate. Also, being right next to the cab is also compounding this effect, putting metal next to the antenna like in the picture also requires lengthening the antenna to compensate. These two aspects are working together to require this length change.
Does this mean that you have a problem? No. You compensated for the problem that you found by lengthening the antenna. I wouldn't worry to much about it at this point, except.
If I may make a recommendation, two springs, even if heavy duty, may cause enough sway in the antenna that it hits the cab of your truck, and will definitely cause more sway than one. Depending on the spring, one may be enough to cause the antenna to hit the cab as well. If either of these gets to be a problem, I would look into replacing one or both of those springs with a riser.
The DB
You are correct, I should have used the word minimize.
I guess I should explain in greater detail, I use to run cophased fiberglass sticks thus the location of my mount. I switched over to a whip “102” about a year ago and just added springs spacers and washers to achieve to best swr and close as possible to 50ohm on channel 20.
Then I bought 108” whip and added a spring still to short, and added a 1 1/2 barrel nut. Still to short.
That’s when I went back an measured my old setup which was 114”
Thanks for your response,
Yes I ran the scan and adjusted to achieve the lowest dip on channel 20.
I used a resent coax at 50ohms which I made to setup all my antennas, length is set by velocity.
My grounds are probably overboard and I was wondering why 108” wasn’t close
Check your antenna with a short jumper and see how much if any difference from your main feedline coax. The true antenna reading is with a short jumper measured at the antennas feed point.
Another thing is a straight whip isn't 50 ohms, it's typically 37 ohms.
Your mounting location can play a role because it's close to the cab and the surrounding metal can detune the antenna some.
On mobile installations, we tend to trade off efficiency with praticality.
Please read:
http://www.k0bg.com
This In my mind is a top site for what you need to know.
I had so meany issues with my S.S.Whips but after the help on this fourm, and the info from K0bg I got right.
The S.S. Whips run an R= around 36-37. and with an X=0, I found that I needed to do a lot of bonding.
I can't tell you where but some place on this forum are photos I have put up of my mfj 259c when I was working out my mobile set up.
Good luck, have fun. There is a lot of good info. here ans folks as well.
I have emailed Alan with questions before, and he's a really cool guy. TONS of good information on his site.
73,
Brett