converting an A-99 to a 1/4 wave 20m ground plane

[mr_fx]

So I was thinking the A-99 is worthless as tits on a boar hog... but the thing is about 16.5 foot long... so I tore mine apart

The top element is around 70" long and the middle is about 68" long... for a total of about 138" when the center and top sections are added together

Then I put the bottom section in a vice and grabbed a 24" long pipe wrench... place the pipe wrench on the metal section and started turning... POP went the element on the inside... lol not gonna lie I was happy, turned it a few more times and was able to get both end caps off the main fiber glass tube...

next on the list is soldering up a 60" long piece of wire to the top cap, then attach it to the feed line inside the A-99

finally I will make a hub for the radials... not sure if I should slope them of not, and still on the fence about weather I should go 1/8 wave or 1/4 wave on teh radials

I will get pics up in a few minutes

 

[mr_fx]

 

[KC9Q]

For a good 50 ohm match use 1/4 wave radials. More specifically, 1/4 wave plus 5%, (234/F(MHz) x 1.05) at a 45 degree down tilt if possible. If the radials are straight out the impedance would be closer to 37 ohms or so.

Mikie

 

[mr_fx]

what do you think the match would be with 1/8 wave radials? someone needs to show me how to use one of the antenna modeling programs... I was mostly trying to avoid 16.5' long radials, but hanging at 45° they would only stick out 8.25' and would hang down 8.25' as well

hmmm... 3 or 4 radials?

 

[The DB]

I would go with 4 radials. 1/8 wave radials will work, but if you can fit them in 1/4 wave radials will typically work better and make the antenna more efficient.

Working with HF frequencies things start to get pretty big don't they.

I don't know that an a-99 would have been my first choice for this project, but whatever works. I'm interested to see your results.


The DB

 

[Captain Kilowatt]

From a shear mechanical standpoint I would rather have eight 1/8 wave radials than four 1/4 wave radials on a 20m groundplane.

 

[mr_fx]

From a shear mechanical standpoint I would rather have eight 1/8 wave radials than four 1/4 wave radials on a 20m groundplane.

please explain the math on this... I really would like to know why, not just know

 

[W5LZ]

"From a shear mechanical standpoint"... mechanical, not electrical... same are but half the space?

 

[Captain Kilowatt]

please explain the math on this... I really would like to know why, not just know

Get in the back of a pickup truck and have a friend drive. Hold a piece of aluminum radial material that is 8 feet long and see how hard it is to hold as the truck picks up speed. Now do the same with one that is twice as long. Notice the difference in drag forces. No math needed, just a good sense of mechanical engineering.

BTW the old radio Shack 1/2 wave groundplane for CB used 1/8 wave radials. many short radials are better than just a few longer ones especially if the GP is elevated rather than ground mounted.

 

[mr_fx]

so maybe 8 radials at 1/8th wave?

 

[mr_fx]

I missed measured something... looks like it is a little short... 3" short

but those figures are for bare wire... so how would fiberglass effect the figures?

that being said i can still make a riser to go between the top and middle section and make up the 3" that way... I have barrel connectors laying around that fit the fitting... or I could install a 4 foot firestik and lop off the top loaded section...

or i could just open up the top of the top section and extend it a few inches...

 

[mr_fx]

seems like eight (8) 102" whips would be perfect for this... of course this would no longer be a 100% free antenna

that being said i already have a hub built for 6 radials @ a 45° rake, so maybe I will just use that, or I could just build another hub with 8 radials

 

[binrat]

seems like eight (8) 102" whips would be perfect for this... of course this would no longer be a 100% free antenna

that being said i already have a hub built for 6 radials @ a 45° rake, so maybe I will just use that, or I could just build another hub with 8 radials

If you got nothing into it then try the 6 radial one. The try the 8 radial one as well.

 

[mr_fx]

I have enough scrap laying around to make a hub for 8 radials

 

[mr_fx]

since the wire is encased in fiber glass that should change the velocity factor making a 1/4 wire actually be too long and it would need be shortened, so 3" is 1.515% short... so it might just work out perfectly without a riser