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adjusting SWR and hamsticks - help??

Nov 3rd 2018, 12:07

KM6SQB

Joined: May 4th 2018, 11:28
Total Topics: 0
Total Posts: 0
Hi all, just got my general 2 months ago! Have Yaesu 450 D and trying to tune MFJ 20 meter hamsticks. Have dipole setup(2 sticks), and unable to tune with BOTH whips inserted into fiberglass portion! When I leave one whip out, other inserted about 6" into fiberglass, SWR reads just over 1.5:1, and Yaesu int ant tuner tunes it! Results go contrary to what I believe hamstck dipoles should do. Also, when I install a 1:1 balun on ant coax, the int Yaesu tuner will NOT tune, but with balun removed from line, the int ant tuner tunes easily. I am a pediatrician, and just like trying to figure out why a child is not gaining weight normally, this has got my neurons going in circles!! Any help would be much obliged! thanks in advance!!
Nov 3rd 2018, 14:43

N2VHZ

Joined: Apr 22nd 2012, 18:58
Total Topics: 0
Total Posts: 0
These quotes are a conversation copied from Eham addressing the problem. but, here are my 2 cents...perhaps one of the antenna are defective, or the entire setup is too close to the ground (earth) or something that is in the "near field" reacting with it?
You know, the internal tuners on radio only tune a 3:1 (150 ohms) into a usable range, whereas an external tuner, such as the MFJ-939 auto is excellent at curing the problem. Most seasoned hams will say that you have to solve the problem at the antenna end, to obviously have full radiation power, (with a true 1:1 SWR ratio), but the alternative would be what I suggested when you cannot solve the issue at the antenna. An auto tuner will be a bonus to have during the entire time you are in the hobby! And congratulations for getting into the hobby!!!
I lived in many austere and compromised locations and conditions and found that with the availability of many bands, and modes, sometimes an adjustment of operating worked the best. So when I couldn't use an HF band because of antenna issues, I switched my needs to 6 and 2 meter SSB operating and found that it fulfilled my needs. Good luck!!!! Ken - N2VHZ
Nov 3rd 2018, 14:48

N2VHZ

Joined: Apr 22nd 2012, 18:58
Total Topics: 0
Total Posts: 0
Quote by N2VHZ


I am just getting back in to ham radio after a 20 year absence.

I needed a quick, non-wire portable HF antenna for 20M -10M (the staff at the local park doesn't like
the idea of a support rope attached to their trees). I decided to go with some MFJ hamstick style radiators ,the MFJ-347 dipole mount and a 15ft fiberglass pole I had on hand.

I know the 15ft pole is rather short, but I got the 10M and 15M sticks to tune up nicely. I did have to offset the lengths of one stinger by 1/4"-3/8" compared to the other side to get the best SWR (around 1.2:1) on both bands.

On 20M I can't get anything bellow 2:1 for SWR. I checked the sticks for continuity between the base threads and stingers. I pulled the stingers and checked for the resonant point of the coils alone which came out around 27.3 Mhz with a 1.3:1 SWR. I tried several different lengths and types of coax with no improvement.

I then tried offsetting the length of 1 stinger- I got the SWR down to 1.15:1 but about 7" of one stinger was down inside the coil which isn't allowed. Obviously something is unbalancing the antenna. Is the 15' height just too low for 20M or perhaps the antenna is coupling to the coax?

Thanks,

Mark
KE8DIH


Nov 3rd 2018, 14:50

N2VHZ

Joined: Apr 22nd 2012, 18:58
Total Topics: 0
Total Posts: 0
Quote by N2VHZ


If you plan on working 15m and 10m on that antenna too, I'd set it back to normal and pick up a decent tuner to work 20m with. If you place the tuner right at the feedpoint of the antenna (which you totally can do in your scenario) there will be negligible loss and you might even be able to get 40m, 17m, and 12m out of it too.

I used the MFJ hamstick dipole years ago and found the same problem with it; at 1.4wave up you have to set one stinger in much shorter. If you decide to keep it that way (or anyone else reading this) be sure to clip the extra length from the stinger. If you don't, the loading coil will pump power into the stinger inductively and eddy currents will eventually destroy the antenna. I found this out the hard way.


Nov 3rd 2018, 14:52

N2VHZ

Joined: Apr 22nd 2012, 18:58
Total Topics: 0
Total Posts: 0
Quote by N2VHZ


If the coax runs straight down from the feedpoint to the ground, it shouldn't be coupling much to the antenna.

Possibly something's wrong with one of the antennas; however at 15' above ground on 20m the ground influence is pretty powerful and you may never achieve a good match to 50 Ohms. That would occur at about 32-33 feet above ground.

A VSWR of 2.0 isn't bad, I'd have no problem dealing with that and just using it.



Nov 3rd 2018, 14:58

N2VHZ

Joined: Apr 22nd 2012, 18:58
Total Topics: 0
Total Posts: 0
Quote by N2VHZ

Did you try mounting the dipole in a vertical orientation? This would help with your height above ground situation and lower your takeoff angle. This gentleman made it work quite well:

https://youtu.be/CbQwLPVmG70
Nov 3rd 2018, 15:27

N2VHZ

Joined: Apr 22nd 2012, 18:58
Total Topics: 0
Total Posts: 0
Quote by N2VHZ


I am just getting back in to ham radio after a 20 year absence.

I needed a quick, non-wire portable HF antenna for 20M -10M (the staff at the local park doesn't like
the idea of a support rope attached to their trees). I decided to go with some MFJ hamstick style radiators ,the MFJ-347 dipole mount and a 15ft fiberglass pole I had on hand.

I know the 15ft pole is rather short, but I got the 10M and 15M sticks to tune up nicely. I did have to offset the lengths of one stinger by 1/4"-3/8" compared to the other side to get the best SWR (around 1.2:1) on both bands.

On 20M I can't get anything bellow 2:1 for SWR. I checked the sticks for continuity between the base threads and stingers. I pulled the stingers and checked for the resonant point of the coils alone which came out around 27.3 Mhz with a 1.3:1 SWR. I tried several different lengths and types of coax with no improvement.

I then tried offsetting the length of 1 stinger- I got the SWR down to 1.15:1 but about 7" of one stinger was down inside the coil which isn't allowed. Obviously something is unbalancing the antenna. Is the 15' height just too low for 20M or perhaps the antenna is coupling to the coax?

Thanks,

Mark
KE8DIH


Nov 4th 2018, 10:50

KM6SQB

Joined: May 4th 2018, 11:28
Total Topics: 0
Total Posts: 0
thanks for your replies; if you don't mind a few more "silly" or self-evident questions?

As I read, even though the SWR may read 3:1 or higher, if auto antennae tuner can find solution and tune it, then may assume that SWR is probably 1.5:1 or less and good to go?

Yes, RE hamstick dipole, one end has fiberglass alone(no whip), and other end has whip inserted into fiberglass about 7" too, so will definitely cut the excess internal whip to avoid heat injury.

Re resonant frequency, how do you find it? my SWR measures of course, SWR and power only, no frequency, and what I did was find a midpoint in the 20 meter band and then with each few second transmission, would move the whip further in or out of the fiberglass portion to find the best ratio. Also, to make sure I did it right, I put the transceiver at around 25 watts am, then moved the SWR to full deflection, then put it in mode to measure SWR. Is this correct? or should I put out the full 100 watts, then the SWR meter would show full deflection(and then some), but that would artificially cause a higher SWR reading when I place the meter in SWR reading mode(I found that out couple days ago), and the MFJ SWR instructions said nothing RE the amt of emitted power needed as long as full scale deflection showed. thanks again all!!!

Harry KM6 SQB (Huntington Beach, CA)
Nov 5th 2018, 10:44

WA0CBW

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
In most cases the resonant frequency will have the lowest SWR. Also you must re-calibrate the SWR meter for each change in power. SWR is the ratio of forward voltage (or power) to reflected voltage (or power). If you are measuring at the transmitter end of the coax use a piece of coax that is 1/2 wave length long at the frequency you are measuring. Don't forget to account for the coax loss in your calculations.
Bill

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