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discone antenna modification

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steve2004

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I had bought an antenna model: MFJ-1868 Discone
Xttp://www.mfjenterprises.com/products.php?prodid=MFJ-1868


I understand the frequency Wideband: 25-1300 MHz

What I want to do is "Trim" the antenna to have a start (low frequency) point
of 120Mhz instead of 25Mhz
How much do I need to "Trim"
from 25Mhz to 119Mhz is not needed.



could you guide me thru what needs to be cut?
I mainly need to monitor Air-band - Emergency services on 140-160 400-470 and 800Mhz

btw Is this a good antenna for a scanner?
(seems a little weak in the 800-900mhz)

Thank you.
 

LarrySC

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This may not work for you but I have a discone that is just flat top, no vertical. The elements on top were about 8 to 12". I dont remember the exact. I marked one at just under 3" for 1/4wave 800 and all the others in a spiral to the last 8" element which was not trimmed. Each element then is a dif length. This made it broadband from VHF thru 800. You need to study the formula for discone antennas. They operate at LESS than 1/4wave across the aircraft band. Some discones are made with a piece of "rod stock" for the center hardware where the top elements mount. This can be replaced with a taller piece of 3/8-24 thread stock which will mount a whip or other element. Good Luck.
 

steve2004

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LarrySC said:
This may not work for you but I have a discone that is just flat top, no vertical. The elements on top were about 8 to 12". I dont remember the exact. I marked one at just under 3" for 1/4wave 800 and all the others in a spiral to the last 8" element which was not trimmed. Each element then is a dif length. This made it broadband from VHF thru 800. You need to study the formula for discone antennas. They operate at LESS than 1/4wave across the aircraft band. Some discones are made with a piece of "rod stock" for the center hardware where the top elements mount. This can be replaced with a taller piece of 3/8-24 thread stock which will mount a whip or other element. Good Luck.
I am confiused...



I attached a pictures of the antenna I bought.
So you are saying I should cut out the Upper-whip and the radial elements?

the top whip is 39" 1/2
the Radial are 36"

or if I don't use the radial will it perform as well?
there are X8 12" vertical Elements that should be touched right?

Thanks
 

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traumacop

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Remove the vertical whip with the black loading coil. That element is used for the low band operations. Some manufacturers sell the discone with and without the vertical low band option.
 

prcguy

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Darrin is right, loose the top whip. I tested the Diamond, RS and Kreco Discones years ago with a network analyzer and found they all went down to at least 120MHz and I think the Kreco started in the 80MHz range. The top whip on the Diamond and RS resonate sharply at about 50MHz only. You can measure the length of the skirt radials and calculate the lowest useable frequency that will provide a reasonable match. 234 divided by frequency will give the length in feet and 10ths of a foot and its fairly accurate for a discone. For a lower cutoff frequency of 120MHz, the skirt length would have to be about 1ft, 11 1/2 inches. The claim of 25-1300MHz by most manufacturers is an outright lie unless you consider any piece of metal at the end of the coax will pick something up.
prcguy
 

steve2004

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prcguy said:
Darrin is right, loose the top whip. I tested the Diamond, RS and Kreco Discones years ago with a network analyzer and found they all went down to at least 120MHz and I think the Kreco started in the 80MHz range. The top whip on the Diamond and RS resonate sharply at about 50MHz only. You can measure the length of the skirt radials and calculate the lowest useable frequency that will provide a reasonable match. 234 divided by frequency will give the length in feet and 10ths of a foot and its fairly accurate for a discone. For a lower cutoff frequency of 120MHz, the skirt length would have to be about 1ft, 11 1/2 inches. The claim of 25-1300MHz by most manufacturers is an outright lie unless you consider any piece of metal at the end of the coax will pick something up.
prcguy


For a lower cutoff frequency of : 234/ 100Mhz = 2.34' (since airband is lower than
120 I decided to lower the usable freq to 100mhz)
how much do I need to remove of 36"






Sorry anout my ignorance! I rather fix it now on a test tripod, than on a 65' tower....
 
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prcguy

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If you shorten the cone you have to shorten the top hat to keep the ratio the same, are you sure you want to do this? The top hat is between .67 and .70 diameter of the skirt length depending on what book you look in. I would leave the antenna a bit longer than my lowest operating freq unless you have some equipment to measure return loss as you trim it. I would not attach any whip to the top. The original whip is base loaded and resonant at 50MHz and the coil acted like a choke in the VHF/UHF bands, in other words it probably had little effect on the VHF/UHF performance. If you put a vertical UHF whip on it could actually degrade UHF if radiation from the whip is not in phase with the rest of the Discone. One positive benefit of shortening to 120MHz at the low end is the upper end will be more in line for 800MHz reception. A Discone has a frequency range of about 8:1 when properly designed. The radiation pattern gets a little squirrelly at the top with many lobes and making the low end about 120MHz would push the top end up to the mid 900s.
prcguy

steve2004 said:
For a lower cutoff frequency of : 234/ 100Mhz = 2.34' (since airband is lower than
120 I decided to lower the usable freq to 100mhz)
how much do I need to remove of 36"






Sorry anout my ignorance! I rather fix it now on a test tripod, than on a 65' tower....
 

steve2004

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prcguy said:
If you shorten the cone you have to shorten the top hat to keep the ratio the same, are you sure you want to do this? The top hat is between .67 and .70 diameter of the skirt length depending on what book you look in. I would leave the antenna a bit longer than my lowest operating freq unless you have some equipment to measure return loss as you trim it. I would not attach any whip to the top. The original whip is base loaded and resonant at 50MHz and the coil acted like a choke in the VHF/UHF bands, in other words it probably had little effect on the VHF/UHF performance. If you put a vertical UHF whip on it could actually degrade UHF if radiation from the whip is not in phase with the rest of the Discone. One positive benefit of shortening to 120MHz at the low end is the upper end will be more in line for 800MHz reception. A Discone has a frequency range of about 8:1 when properly designed. The radiation pattern gets a little squirrelly at the top with many lobes and making the low end about 120MHz would push the top end up to the mid 900s.
prcguy

I will just remove the black coil/vertical whip.
and leave it as is then.


Thank you.
 

kb2vxa

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Hi Steve and all,

Some "antenna experts" sure know how to destroy in a minute what RF design engineers spent decades developing. You could have done much better than MFJ with the much sturdier Diamond, a bit more pricey than Mighty Fine Junk. Oh well it works so leave it alone, just when it falls apart replace it with something that'll hold together. Most discones are made to transmit on ham bands but are "broad" enough to receive reasonably well across the board and you'll see the optimum points by scrutinizing the electrical specifications, they're all pretty much the same in that regard.

Now here's the rub, those with the vertical whip come in two flavors, one optimized for CB and the other for 6M ham use. The latter is more easily retuned for VHF Lo Band and both are made to be optimized in that area alone, but in both cases as with those without it's a "no tuning allowed" situation, slicing up fixed elements can only louse it up unless you use specialized lab equipment and scientific method. They're made the way they are for a reason and those that think they know better than a flock of MSEEs and PhDs are thier own fools. I could go on boringly with the whys and wherefores but please guys, bear one thing in mind, if it ain't broke don't fix it.
 

esu78

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Steve
any luck with your antenna mods? I have the same MFJ, and i noticed too its "weak" on 800Mhz. Have you noticed any improvements?

Thanks !
John
 

timmer

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So, if I wanted to increase my 30-50 mhz reception on my discone, could I put a longer top element in my discone? (That is, if I were able to find one that fit it.) Or would that mess up the rest of the reception?
 

Al42

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The discone is a simple antenna, guys. Make the bottom half of a ground plane for the lowest freqency you want (1/4 wavelength at that frequency). Make the disk diameter .7 the length of a ground plane element. The antenna will work fairly well (about 2db loss over a ground plane or dipole) from the lowest frequency to around 3 or 4 times that frequency. No matter what you do you aren't going to get a discone to perform over more than a 4:1 frequency range. You can design it for a higher frequency and add a vertical radiator for a lower band, making the antenna a ground plane in parallel with a discone, but the discone only works over a 4:1 ( theoretical maximun - practically it's closer to 3:1) frequency range.

None of which matters for reception since a) you obviously aren't going for gain if you're using a discone and b) any conductor stuck in the air will receive some signals.
 

steve2004

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esu78 said:
Steve
any luck with your antenna mods? I have the same MFJ, and i noticed too its "weak" on 800Mhz. Have you noticed any improvements?

Thanks !
John

I cut X4 out of the X8 vertical 12" whip to 6" now it looks like a X of 6" and a X of 12"
and I notice the 800Mhz works better. still without thevertical whip.

I may have ruin it? I don't know but I know 800mhz seems better now.

as per the cone (whip) I don't want to vut them until I know the proprer lenght for the freq I need.

Cheers.
 
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