Diamond D130J Top Element

[invergordon]

Is there any mileage in trimming the top loading coil element to enhance the reception at 123.7MHz?

 

[jonwienke]

No, the top element is for receiving low-band 25-50MHz. The discone itself receives ~100MHz on up.

 

[prcguy]

The top whip on a DJ130, RS or similar Discones is tuned around 50MHz. It does sort of enhance reception in the VHF lo range but compared to where it resonates at 50MHz the rest of the VHF lo band can be 10 or 20dB down in performance.

If you will never use the Discone below the VHF air band, I suggest you remove the top whip. A Discone was never designed with a top whip and it may have an adverse affect at some frequencies in the 100 to 800MHz range where the native Discone is operating.
prcguy

 

[craneoperator99]

I have a similar question. When I put my new Diamond D130J up it was recieving VHF low band fine. However, few hours after putting my new antenna up the lo-band signals suddenly disappeared except for the very strongest. Above 100 MHZ everything seems to work fine. Only VHF low band was affected. This has led me to conclude that something happened to the low band whip on top. Although I most likely will not pull the antenna down after going through all the trouble of putting it up, especially since low band isn't nearly as important to me as some of the other bands, I still would like to know if anyone else has experienced this. I would also like if anyone knows any possible solutions or why this happened to me. Thanks!

 

[toastycookies]

It could be as simple as a loose screw. Honestly you will not figure it out until you look at the antenna physically.

 

[n4yek]

However, few hours after putting my new antenna up the lo-band signals suddenly disappeared except for the very strongest.

Perhaps when you installed the antenna, the band was open and then it closed on you.

Just a thought.

 

[craneoperator99]

It could be as simple as a loose screw. Honestly you will not figure it out until you look at the antenna physically.

Thanks, I guess I'll have to examine it carefully next time I pull it down.

Perhaps when you installed the antenna, the band was open and then it closed on you.

Just a thought.

Unfortunately, I don't think that's the case. My Diamond is now being outperformed on low band by a ham antenna.(2 meters/70 centimeters)That same antenna was performing a lot worse than the discone right after I put it up. I should have also mentioned in my original statement that the agencies I was listening to were local.

 

[ko6jw_2]

I have done some experimentation with the D130 on low band. First, this antenna is not a discone on low band - it is a sort of base loaded ground plane. As was pointed out by prcguy, it tunes up at about 50Mhz and can be trimmed to work a little higher in the six meter band. The bandwidth is very narrow on low band - maybe about 4Mhz at the most. Here in California the main low band system is the CHP. They use frequencies from 39Mhz to 45Mhz with most of the activity around 42Mhz. The stock D130 is useless at these frequencies and the SWR is extremely high. I replaced the stock whip with one that is 48 inches long. The antenna is now resonant at about 42.5Mhz and does a good (not excellent) job with CHP. You don't say what portion of the low band you listen to, but a longer whip may be the solution.

The whip is 0.10 inch. They can be obtained from many radio supply outlets in various lengths.

I am considering building a 42Mhz j-pole. Rather long, but I think it will be effective. I had a 42Mhz ground plane for a while and it worked very well.

 

[prcguy]

The BW of the base loaded top whip is really narrow, hardly 1MHz wide at around 50MHz if I remember. Lengthening the whip for 42MHz or something even lower is probably fine IF you leave the base loading coil in place. It sort of acts as a choke and decouples the whip from the DIscone at some higher frequencies and will help shorten the whip to keep it within some safe mechanical limits. Stick a long whip on a Discone without a properly designed coil at the base and you'll not have a Discone anymore, it will be a random size coat hanger in the air.

Otherwise I would simply check the suspected coil with an ohmmeter from the top disc to the whip and bang on the coil with the meter connected to see if there are any intermittent connections under the plastic.
prcguy

I have done some experimentation with the D130 on low band. First, this antenna is not a discone on low band - it is a sort of base loaded ground plane. As was pointed out by prcguy, it tunes up at about 50Mhz and can be trimmed to work a little higher in the six meter band. The bandwidth is very narrow on low band - maybe about 4Mhz at the most. Here in California the main low band system is the CHP. They use frequencies from 39Mhz to 45Mhz with most of the activity around 42Mhz. The stock D130 is useless at these frequencies and the SWR is extremely high. I replaced the stock whip with one that is 48 inches long. The antenna is now resonant at about 42.5Mhz and does a good (not excellent) job with CHP. You don't say what portion of the low band you listen to, but a longer whip may be the solution.

The whip is 0.10 inch. They can be obtained from many radio supply outlets in various lengths.

I am considering building a 42Mhz j-pole. Rather long, but I think it will be effective. I had a 42Mhz ground plane for a while and it worked very well.