ARRL

Forum Home - Rules - Help - Login - Forgot Password
Members can access, post and reply to the forums below. Before you do, please first read the RULES.

Triband beam probleem

Oct 25th 2011, 00:10

WD5DZG

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I am no antenna expert but I have restored an older tribander, sys33 antenna once sold by Maco in Memphis TN. Fortunately I was able to get parts and it is complete and mounted on my Hazer and ready to go up except for one sticky problem.
SWR sucks on 20 meters. It is fine on ten meters, fine on 15 meters with SWR readings near 1:1 and C(x=O) and L(x=0) however on 20 meters the best I can do is one spot about 50kcs wide where I get 2:1 SWR. Capacative reactance is out of sight with Xc =367Pf, Inductance =.315ul The antenna R =40 and X =29 These readings were taken with an MFJ259B.
Orginally I started with an SWR of 1.6 down around 19.97 Mhz which indicated I needed to shorten the 20 meter elements, As I shortened themm the best resonance point has moved up the band into the 20 meter CW portion but I can no longer get a reading below 2:1. I am not sure what to try next, The low inductance reading on 20 makes me suspect that one of the trap coils may be shorted or have some sort of insulation problem. The proceedure I have been using is to make adjustments and crank it up the tower 25 feet or so and run SWR checks (I have a metal roof) Can anyone point me in a likely direction? Bill WD5DZG
Oct 25th 2011, 00:12

WD5DZG

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The question I posed has a typo, it should read 13.97 Mhz not 19 mhz, sorry
Bill WD5DZG
Oct 25th 2011, 05:15

KB0HAE

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Hi Bill. I am no expert either! Jus a few thoughts. How tall is the tower? Have you tried the antenna at the full height? At 25 feet how close is the metal roof to the antenna? Are there any other metal objects nearby? I would try pointing the antenna in different directions to see if the SWR changes.

Good Luck!
Oct 25th 2011, 12:20

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

If a trap was shorted out it is unlikely that 10 and 15 meters would have such low SWRs. It certainly sounds like you have some interaction with the metal roof, or a nearby antenna. You should expect the SWR to vary significantly as the height above ground is changed. For example, a low dipole adjusted to 1:1 SWR may be expected to go to 2:1 when raised to a height of 1/3 wavelength.

Zack Lau W1VT
ARRL Senior Lab Engineer

Back to Top