High SWR

[Vondutch]

I just put up a Butternut HF9V. I have 36 radials installed via a radial plate as per the manufactures instructions. I also have it grounded at the base to a 10' copper earth grounding rod, I'm using the 75 ohm feedline. My SWR is off the chart on all bands. I cannot figure it out, I have been through the manual a dozen times and can't find anything that I missed. I have been through the ARRL Antenna Book and can't see anything I've done wrong. I suspect my grounding but for the life of me I can't figure it out. Any ideas?

Thanks!!

 

[prcguy]

There is a shunt coil at the base for tweaking impedance on the lower bands but it shouldn't affect 20m or above. Sounds like something might be assembled wrong if all bands are affected. The antenna is designed to be fed with 50 ohm coax and 75 ohm coax should have a slight affect on the match on all bands.

I put one on a commercial building once that had a 100' X 100' copper sheet metal roof and I had to make some element length adjustments and cut a few turns off the shunt matching coil since the antenna is more commonly installed with only a few radials. The huge copper roof really brought the feed impedance down compared to only using a few radials.
prcguy

 

[Vondutch]

There is a shunt coil at the base for tweaking impedance on the lower bands but it shouldn't affect 20m or above. Sounds like something might be assembled wrong if all bands are affected. The antenna is designed to be fed with 50 ohm coax and 75 ohm coax should have a slight affect on the match on all bands.

I put one on a commercial building once that had a 100' X 100' copper sheet metal roof and I had to make some element length adjustments and cut a few turns off the shunt matching coil since the antenna is more commonly installed with only a few radials. The huge copper roof really brought the feed impedance down compared to only using a few radials.
prcguy

Thanks for the response. I've been over my assembly a dozen times, and I can't see anything I missed. The shunt I is something I've wondered about. I can't find too much info on adjusting it, I hate to shorten it and then find out I didn't need to.

 

[prcguy]

Very short loaded verticals over a good ground plane usually have a very low feed point impedance and a particular value of inductance across the feedpoint can effectually raise the impedance to match the coax better. The problem is worse on the lower bands where the antenna is really short and usually 20m and above will not have as much loading effect from coils in the traps and will not require external matching.

The shunt coil is tuned for the best match on the lowest band or split between the two lowest bands and its effectively out of the circuit on the higher bands where its not needed.

In my experience with a Butternut HF9V, the stock coil is about right for the antenna being on a pipe in the ground with no radials or on a roof with just a few radials. If the ground plane is better (like a big copper roof) the impedance on the lower bands goes lower and spreading the coil apart will change the match in the right direction.

The ground plane on the huge copper roof I dealt with caused me to remove a few turns and I still had to spread the coil apart some to get the match down on the lowest band. BTW, you want as much ground plane under a vertical as possible and your 36 radials is a good start. People who simply stick that antenna on a pipe in the ground with no radials might have a good match but the performance will be way down compared to your 36 or more radials.
prcguy

Thanks for the response. I've been over my assembly a dozen times, and I can't see anything I missed. The shunt I is something I've wondered about. I can't find too much info on adjusting it, I hate to shorten it and then find out I didn't need to.

 

[kc0kp]

I just put up a Butternut HF9V. I have 36 radials installed via a radial plate as per the manufactures instructions. I also have it grounded at the base to a 10' copper earth grounding rod, I'm using the 75 ohm feedline. My SWR is off the chart on all bands. I cannot figure it out, I have been through the manual a dozen times and can't find anything that I missed. I have been through the ARRL Antenna Book and can't see anything I've done wrong. I suspect my grounding but for the life of me I can't figure it out. Any ideas?

Thanks!!

Two suggestions:
Have a friend take a fresh look at the assembly. Heathkit used to recommend this for kits that did not work. The second person is less likely to make the same mistake if one was made.
Second see if you can borrow or buy an antenna analyzer. You can then sweep to see if and where the antenna is resonant If it is resonant and just outside the bands you want, then start tuning away at it. If there are not points where it is flat near the ham bands, I would suspect an assembly error or a bad chunk of coax feeding the antenna.
Craig

 

[WA0CBW]

Put a dummy load at the antenna end and check the SWR. Check the coax and connectors for an open or short.
BB

 

[majoco]

I'm using the 75 ohm feedline.

The 75ohm feedline from the antenna is a matching section only - you should be using 50ohm cable back to the transmitter. You can connect your VSWR meter at the 50/75ohm junction but your transmitter wants to see 50ohms.

 

[ab3a]

Getting a high SWR on all bands is an indicator that the problem is likely very close to your radio. Check your connector at the radio.

Also, if you don't have a dummy load, at least pull the coax off of the antenna and measure the continuity from end to end with a clip lead. It should read open, and then when you put a clip lead at the far end you should see a short.

If you pass all of these basic tests, THEN go looking for assembly problems at the antenna.