Antenna and Coax questions

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SCPD

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Two seperate questions resulting from discussions at the last ham club meeting I've been to:

1. Using 75 ohm coax to transmit. Although it's something I would never do, the discussion centered around the match you would get. If I have a 1:1.5 match with 50 ohm feedline, is the impedence then either 33.3 or 75 ohms? And if that is true, then if I was to use 75 ohm coax, would I just get a 1:1.5 match? You see where the discussion went.

2. Half wave antenna on a fiberglass van. Let's say I drill a hole in fiberglass, like the top of a van, or maybe a boat, or even a piece of wood, in other words, something with no ground plane. And I put a half wave whip on it. Should this play just fine?

Come spring, I'm gonna put new batteries in the MFJ analyzer and play with both scenerios myself, but while I'm nice and cozy with the heater, I thought I'd throw out this for discussion.
 

prcguy

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Using 75ohm coax with an antenna that presents a 50ohm load will cause a slight mismatch but its not life threatening and should not hurt your radio. Calculating the additional loss is complicated and depends on many factors.

An end fed half wave antenna does not need a counterpoise to work against and is popular for mounting on fiberglass vehicles. Problem is a half wavelength end fed antenna presents a very high impedance at the feedpoint and some sort of matching is required, you just can't cut a whip to 1/2 wavelength and connect the coax.

If you look at commercially made 1/2 wave antennas for fiberglass vehicles and boats you'll see there is a coil or something at the base where the matching components live.
prcguy
 

kb2vxa

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"Using 75ohm coax with an antenna that presents a 50ohm load will cause a slight mismatch but its not life threatening and should not hurt your radio."

That assumes the load is actually 50 ohms resistive, reactance tends to upset the apple cart. You'll never know unless you measure it at the load, transmission line adds its own variables and the reason why specific lengths of specific impedance lines are used as transformers. Heh, this is a lovely can of worms for general discussion when you need a Masters degree to understand the finer points.

"You see where the discussion went."
Yeah, it happens right here all the time and winds up with balloon heads on one side and pins on the other. (;->)

"An end fed half wave antenna does not need a counterpoise to work against and is popular for mounting on fiberglass vehicles."
Sorry OM but it does as it is so with any end fed radiator. All a 1/2 wave or 5/8 wave end fed does is raise the 90 degree (current) point higher up the radiator thus lowering the vertical radiation angle. It still needs a counterpoise to "complete the circuit", perhaps you're thinking of a few center fed half wave antennas sold just for the purpose where a ground counterpoise is unavailable. I used to see them around mounted on the mirrors of big rigs and behind the air dam when fiberglass came along, don't know if they're still around since 2M isn't so popular with truckers anymore.

"Half wave antenna on a fiberglass van."
I've seen a few get around that problem by installing radials under the roof assuming you're talking VHF here but the wavelength is irrelevant in any case. Where HF is concerned mounting the antenna on the bumper or very low on the body with the coax shield grounded to the vehicle frame is how it's done.
 

lmcsolutions

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Home Antenna

Hey guys quick question since I am new to all of this and any help would be great. Im looking to purchase an antenna and cable to connect to my home scanner.. just a portable RS Pro95. I live in the Medford NY area and looking to get about 30 miles to Nassau county if that is possible with this scanner. Like I said I am new and just learning. If it is possible please let me know what I would need to make this happen. I would like to go out and purchase it as soon as possible.. thank you all again
Louie
 

prcguy

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An end fed half wave has an identical current distribution and radiation pattern as a center fed half wave (common dipole). So your saying a center fed dipole needs a counterpoise?

You can excite and feed a 1/2 wavelength piece of wire in the center with low impedance coax (you obviously have to break the connection), at the end with a very high impedance matching network and anywhere in between with proper matching and the radiation pattern remains the same except for slight interaction of the feedline.

Where is the counterpoise in the classic J-pole? If you were to install some it would throw off the matching by introducing a lot of capacitance near the very high impedance feedpoint. Look in any antenna handbook or model a half wave antenna fed at different points with EZNEC and lets see if your opinion changes.
prcguy



"

"An end fed half wave antenna does not need a counterpoise to work against and is popular for mounting on fiberglass vehicles."
Sorry OM but it does as it is so with any end fed radiator. All a 1/2 wave or 5/8 wave end fed does is raise the 90 degree (current) point higher up the radiator thus lowering the vertical radiation angle. It still needs a counterpoise to "complete the circuit", perhaps you're thinking of a few center fed half wave antennas sold just for the purpose where a ground counterpoise is unavailable. I used to see them around mounted on the mirrors of big rigs and behind the air dam when fiberglass came along, don't know if they're still around since 2M isn't so popular with truckers anymore.

"Half wave antenna on a fiberglass van."
I've seen a few get around that problem by installing radials under the roof assuming you're talking VHF here but the wavelength is irrelevant in any case. Where HF is concerned mounting the antenna on the bumper or very low on the body with the coax shield grounded to the vehicle frame is how it's done.
 
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