Antenna tuning question

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Tim-B

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If I have some commercial HT antennas with BNC connectors and I want to cut one of them, say like an Antennex DEXC 420 BNX to the proper length for quarter wave at 770 MHz for a scanner antenna

and

If I calculate the length required as:
186,282 X 5,280 X 12 / 770,000,000 / 4 = 3.832 inches for quarter wave at 770 MHz

Then

Do I measure the 3.832 inches from the bottom of the little pointy pin in the middle of the BNC connector where it goes into the tiny hole in the middle of the scanner's connector or do I measure it from just above the connector or does the bottom measure start somewhere else? I guess what I mean is does that little pointy pin in the connector function electrically as part of the antenna or no?
 

Tim-B

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This is the 'tenna. It is about 6 inches long but I can cut it with a heavy duty set of wire cutters. The cap on the end with the red stripes comes off if you pull and twist hard enough. I can pull the cap off the part I cut off and push it onto the newly cut end. The connector is a BNC with a rubber skirting around it for a neater cleaner appearance

 

jonwienke

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Your antenna probably has coiled wire wrapped around a flexible core, so forget trying to use a formula to calculate length. HT antennas are pretty much always electrically "longer" than their overall physical length. Essentially the entire antenna is a loading coil.
 

Tim-B

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Okay, yeah I've seen those coiled ones like dual band ham 'tennas that are quarter wave in one band and half wave in another but this one is very skinny so there is prolly no coil inside.
 

prcguy

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Your particular antenna is more than likely a straight steel cable wrapped in rubber and has no loading coil. I've had a number of those apart. The antenna would start above the grounded part of the BNC but a calculation for length will only get you close, you need to tune it with some test equipment.
prcguy


This is the 'tenna. It is about 6 inches long but I can cut it with a heavy duty set of wire cutters. The cap on the end with the red stripes comes off if you pull and twist hard enough. I can pull the cap off the part I cut off and push it onto the newly cut end. The connector is a BNC with a rubber skirting around it for a neater cleaner appearance

 

ab5r

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Forgive me, but what do you hope to gain from cutting? This is a receiving antenna; not for transmitting. Do you really think your ears would discern a difference? I don't see the WHY.
 

prcguy

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The stock length antenna for what looks like 420MHz would be very close to 1/2 wavelength around 800MHz and a very high impedance. I suspect you would notice a difference between that and a tuned 1/4 wave.
prcguy

Forgive me, but what do you hope to gain from cutting? This is a receiving antenna; not for transmitting. Do you really think your ears would discern a difference? I don't see the WHY.
 

wtp

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for receiving.

just use it as is.
even a paperclip pushed into a bnc will work.
when the 800 band started i did that for a friend that was having trouble with the old multi-band antenna, it was for 30 to 512. it did work better but as he said "i can't walk around like this"
my 800 Mhz radio shack does great for the space station on 145.8 but the 150 motorola antenna is slightly better.
 

mmckenna

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You could try slicing the antenna coating off and see what the base looks like.
To cut for 1/4 wave, you'd measure up from where the part connected to the outer shield stops.

You'd sacrifice one antenna for it, but at least you'd know.

Other way to do it would be to put it on an antenna analyzer, but I'm guessing you don't have one?
 

Tim-B

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Well, doggit, with all the unknows I might just measure from just above the connector and cut it and replace the end cap and leave it alone after that for better or worse.
 

Tim-B

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Ahh, the dark art of RF tech. I bought this bitty 'tenna for my 396XT mainly because it looks really cool on the radio and I was hoping it would work well. I put it on the radio and the radio was almost completely deaf. So I took the thing apart and found that under the rubber cover was a coil of thin wire the bottom of which wrapped into threads on the connector. I saw hole in the top center of the connector so I cut three strands of speaker hook up wire the same length as the rubber cover and cut 1/8 inch of insulation off the ends of them and stuck the bare ends into the hole. I replaced the original coil and the rubber cover and now the thing works awesomely on the scanner. It went from zero bars to five bars on the 700 MHz site I monitor most often and it works very well on the road as it goes from site to site. Works well on FRS and GMRS frequencies too. I love the way it looks on the radio and it is a good size match for that radio.

 

prcguy

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I have one that looks like that and its tuned for 146/440 MHz bands. Its a lousy performer but good for short range like keeping in touch with friends on 2m at a swap meet, etc. It would have very poor performance on 700 or 800MHz and your mod probably came close to adding a 1/4 wave near the 800 band.
prcguy

Ahh, the dark art of RF tech. I bought this bitty 'tenna for my 396XT mainly because it looks really cool on the radio and I was hoping it would work well. I put it on the radio and the radio was almost completely deaf. So I took the thing apart and found that under the rubber cover was a coil of thin wire the bottom of which wrapped into threads on the connector. I saw hole in the top center of the connector so I cut three strands of speaker hook up wire the same length as the rubber cover and cut 1/8 inch of insulation off the ends of them and stuck the bare ends into the hole. I replaced the original coil and the rubber cover and now the thing works awesomely on the scanner. It went from zero bars to five bars on the 700 MHz site I monitor most often and it works very well on the road as it goes from site to site. Works well on FRS and GMRS frequencies too. I love the way it looks on the radio and it is a good size match for that radio.

 
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