5/8 wave @ 37'

[HomerBB]

Just recycled some previously used homebrew parts into another 5/8 wave ground plane antenna. In this instance I decided to go with the Ringo/Maco type loop inductor instead of a coiled spring style. The feedpoint is just over 37' elevation. Seems to be working well.

 

[Marconi]

Homer, good to see you back doing your thing.

 

[HomerBB]

I get pretty restless. The furnture thing we do abdorbs a lot of time, but I wanted to breathe a little on the relax and recreation side. Built an antenna.

 

[Needle Bender]

I get pretty restless. The furnture thing we do abdorbs a lot of time, but I wanted to breathe a little on the relax and recreation side. Built an antenna.

Hey Homer, looks good!
- What length did you choose for your radiator?

 

[HomerBB]

Hey Homer, looks good!
- What length did you choose for your radiator?

22'- 1"

 

[Needle Bender]

22'- 1"

Is that from the point where the ring arm meets the radiator? And is the top arm horizontal like a Sigma II or at an uphill angle like the Maco?

MACO:

 

[HomerBB]

The length is the entire vertical radiator lentgh. About 10" of it is below the radials.
The ring is like the avanti, the "L" brackets are parallel to each other over/under.
Like this:

 

[HomerBB]

Except that my lower bracket is the aluminum plate I mounted my radials on.

 

[HomerBB]

That's the plate, and the radial setup, but not the inductor currently in use.

 

[Marconi]

If you can tell, do you think there is any noticeable difference in the match or responses between these two inductors?

How did you calculate this coil's dimensions?

 

[Marconi]

NB, glad to see you are still with us.

 

[HomerBB]

If you can tell, do you think there is any noticeable difference in the match or responses between these two inductors?

How did you calculate this coil's dimensions?

Howdy, Marconi.

I didn't calculate this coil. I simply used the dimensions that Cushcraft publishes for their antennas. A 13" diameter coil rendered a 41" long tube needed. I did 52" and turned the two ends back toward the vertical for standout brackets.
At this point I can't confidently say I can see a difference. Time will tell.
What I have noticed is that on several occasions I've heard and conversed with some stations with it at 37' that a fellow 4 blocks over from me failed to hear with his A99 mounted at 53'. Might have been a coincidental thing. Don't know...
Also, in the past when tuning a 5/8 antenna at 8' above the ground it worked out easily. I had to raise this one 15' above the ground before I could tune it.
Beyond that, things appeared about the same. On the tuning mast I found my best match with resonance on 27.385 Mhz with a 1.3:1 SWR. At working elevation it reads a 1.1:1.

 

[Marconi]

I did a model and started at 265" inches as you noted. I ended up with 269.7" inches in length tuned using 108" radials however. I could not get the 52"L x 13"D coil to work and show a good match at 27.205 MHz, the antenna was too short and needed more inductance.

I cannot explain this rather large difference in results, but maybe the large plate you had your radials attached to made the difference...that is like a large hub which I did not model.

I tried making the 4" element between the radials and the coil larger and it did improve the model, but I ran into geometry errors before I could get the model to show a good match at resonance. I did not want to take the time to try and model your radial hub plate.

Plus I think your isolation of the bottom of the radiator inside the support and mounting bracket also adds some capacitance I cannot duplicate.

The antenna model shows very good low angle gain and a good match tuned with a 22.5' foot radiator and mounted at 36' feet with my coil at 66"L x .50" diameter tubing and 8" inch legs to this helix axis. However, the band width is pretty narrow covering only a little over the CB band at <2.00:1 SWR so that should provide about as much gain as possible if the old stories about narrow bandwidths is true and easy to determine just using a radio.

The model also shows some CMC on the mast and I did not Isolate the mast or add a choke, but you probably took care of that.

That said however, if I was able to eliminate some of the CMC noted here with either method a choke or isolation...the model would likely show a little less gain as a result.

Keep us posted.

 

[HomerBB]

That's interesting.
I'm not sure how mideling is done, but glad you made the effort.

To be clear on the ring, it is a 1/2" copper tube. The actual length of the circular part is 40". The additional 12 " are the part I bent back toward the vertcal, 6" at each end of the ring, to form the stand-offs for the ring.
I considered the possibility of the plate producing some capacitance, and perhaps the lower end of the radiator.

I went back to my notes to confirm my antenna length. I found no notes of having extended the length frim 22' 1" to 22' 6". However, I may have and did not note it. I'll remeasure it next time i have it on the ground.

Thanks.

 

[Marconi]

I went back to my notes to confirm my antenna length. I found no notes of having extended the length frim 22' 1" to 22' 6". However, I may have and did not note it. I'll remeasure it next time i have it on the ground.

I took your 265" inches to mean from the attachment point of the matching coil on the base of the radiator to the tip.

I know you said something about 10" inches of the radiator down inside of the mount because I cannot duplicate that construction. Your antenna could have been shorter if you tuned higher in the CB band too. My model was done at 27.205 MHz, and I just took a guess the radials were 109" inches, and the radiator average diameter was .875" inches with the radial diameter at 0.60" inches.

How about your <2.00:1 SWR bandwidth...is it narrow?

Attached is an overlay of this model with and without a choke added to minimize feed line currents.