Strange SWR readings

[kd-5-bgt]

I have a Jetstream JTB3 144/440 antenna that I am using for my 2 meter radio.It is about 30 feet off ground to feed point using RG8X coax (mini 8)

At the 2 meter band edges ( 144.100 and 147.900 +/- on each band edge) the meter (MFJ 812B ) shows about 1.4 to 1 at the high power setting on radio but at about center band 146.500 the swr is showing just under 2 to 1.

I can understand if the swr low on one of the band edges it should rise the further you get from that point,but for swr to be lower on band edges than center of the band I find that rather unusal

I have checked the jumper from radio to meter and checked good.Coax is good,grounds in place as they should be.

All connectors are in good shape and tight on radio and meter as they should be

I have checked this with two different radios,Icom 2200H and Yeasu 2900 and the swr shows the same readings on both radios.

I also double checked the meter on my mobil set up just to see if there was something that might not be right with meter ( yea I know MFJ does not make the best equipment but it is all I have for the moment ),Yeasu 2900 into a Larson NMO 150 antenna hard mounted on my pickup.And the swr shows 1.7 to 1 on band edges as mentioned above and 1.3 to 1 center band so I am thinking the meter is fairly close...but the deal with the base antenna has me scratching my head

Any ideas anyone? Thanks in advance !!

 

[Robb]

When you get to the VHF/UHF range, the quality of the coax and it shielding become pretty important.
I don't use mini 8, just LMR coax.

Another possibility is that there might be deviation in the coax center to the outside shielding.
If it is old cable; I would definitely upgrade.

 

[Beetle]

If you can, add about 4 or 5 feet of coax and see if it makes any significant difference. If the antenna isn't VERY close to 50 ohms =/-j0 at the feedpoint, the length of the feedline can be very important.

And as Robb said, larger coax (RG-213 at a minimum; LMR-400 better) should be in your short term plans.

 

[mackmobile43]

Good deals on Andrews Heliax at egay.

 

[Moleculo]

Do you know what type of antenna is under the fiberglass hood? Is it made of multiple cophased vertical arrays? I've seen this type of reading before on some cophased style antennas. I don't know if it's caused by the type of phasing used or the matching system, or something else? I like the suggestion others made about the coax, too.

 

[kd-5-bgt]

Coax is about a year old.Looked it over outside today and did not find any pinched points or broken outer shell anywhere...yea I know mini 8 isnt the best coax to use but was what I had on hand ...

Moleculo I am not sure on the antenna,when I got the antenna new it came to me in two main pieces from the feed point up.Inside the fiberglass housing what I saw was ends of some 12 or 10 gauge unshielded wire that has a coupler with set screws,insert wire from upper piece into coupler ,bottom it out and tighten set screws...only one wire though coming up from feed point...if there is anything else inside the fiberglass shell it is in the lower part of antenna out of sight....

I will continue to play with it as I have time and see if I can locate anything ...so far I have done everything that I can do from the ground and roof.....,will have to lower antenna to have a close look at it

Thanks for the replies guys