When adjusting the antenna length on a dipole to set the vswr, the antenna is shortened or lengthened, is it better to adjust both ends rather than just one? Would this give a better symmetrical signal? Would adjusting the length between the dipoles effect vswr?

Tau.Neutrino said:

When adjusting the antenna length on a dipole to set the vswr, the antenna is shortened or lengthened, is it better to adjust both ends rather than just one? Would this give a better symmetrical signal? Would adjusting the length between the dipoles effect vswr?

transmit or receive? if receive it wont really matter that much, except at really high frequencies.

Bogsnorkler said:

Tau.Neutrino said:

When adjusting the antenna length on a dipole to set the vswr, the antenna is shortened or lengthened, is it better to adjust both ends rather than just one? Would this give a better symmetrical signal? Would adjusting the length between the dipoles effect vswr?

transmit or receive? if receive it wont really matter that much, except at really high frequencies.

Receiving fm on 105 mhz.

Tau.Neutrino said:

Bogsnorkler said:

Tau.Neutrino said:

When adjusting the antenna length on a dipole to set the vswr, the antenna is shortened or lengthened, is it better to adjust both ends rather than just one? Would this give a better symmetrical signal? Would adjusting the length between the dipoles effect vswr?

transmit or receive? if receive it wont really matter that much, except at really high frequencies.

Receiving fm on 105 mhz.

I really wouldn’t worry too much about the swr. as long as they are pretty well equal in length and you have minimal centre gap. the signal is going to be strong anyway so a coathanger would work.

Antennas are made of metal, could an antenna for receiving be made of a magnetic field without using metal?

Tau.Neutrino said:

Antennas are made of metal, could an antenna for receiving be made of a magnetic field without using metal?

Ah, a science fiction novel from the 1950s posed that question. I still haven’t seen a firm answer.

I assume you include semiconductors among the metals here. How do you generate a magnetic field and how do you sense changes in a magnetic field without a metal or semiconductor? Are you thinking of a plasma perhaps? If so, then the answer is yes. See https://en.wikipedia.org/wiki/Plasma_antenna

“A plasma antenna is a type of radio antenna currently in development in which plasma is used instead of the metal elements of a traditional antenna. A plasma antenna can be used for both transmission and reception. Although plasma antennas have only become practical in recent years, the idea is not new; a patent for an antenna using the concept was granted to J. Hettinger in 1919.”

mollwollfumble said:

Tau.Neutrino said:

Antennas are made of metal, could an antenna for receiving be made of a magnetic field without using metal?

Ah, a science fiction novel from the 1950s posed that question. I still haven’t seen a firm answer.

I assume you include semiconductors among the metals here. How do you generate a magnetic field and how do you sense changes in a magnetic field without a metal or semiconductor? Are you thinking of a plasma perhaps? If so, then the answer is yes. See https://en.wikipedia.org/wiki/Plasma_antenna

“A plasma antenna is a type of radio antenna currently in development in which plasma is used instead of the metal elements of a traditional antenna. A plasma antenna can be used for both transmission and reception. Although plasma antennas have only become practical in recent years, the idea is not new; a patent for an antenna using the concept was granted to J. Hettinger in 1919.”

PS, the reason that a 1950s scifi book posed that question is that they wanted to make a stealth spacecraft that was invisible to radar. Metallic components would reflect radar waves. The book, “First Lensman” is a science fiction novel by American author E. E. Smith. It was first published in 1950.

Hey, what do you know, that’s even made it onto wikipedia https://en.wikipedia.org/wiki/Plasma_stealth

“Plasma stealth is a proposed process to use ionized gas (plasma) to reduce the radar cross-section (RCS) of an aircraft. Interactions between electromagnetic radiation and ionized gas have been extensively studied for many purposes, including concealing aircraft from radar as stealth technology. In 1956, Arnold Eldredge, of General Electric, filed a patent application for an “Object Camouflage Method and Apparatus,” which proposed using a particle accelerator in an aircraft to create a cloud of ionization that would “…refract or absorb incident radar beams.”

“During Project OXCART, the operation of the Lockheed A-12 reconnaissance aircraft, the CIA funded an attempt to reduce the RCS of the A-12’s inlet cones. Known as Project KEMPSTER, this used an electron beam generator to create a cloud of ionization in front of each inlet. There are claims that a system was offered for export by Russia in 1999.”

if it’s an end-fed dipole the question’s redundant

but of folded or half-wave centre fed it keeps the radiation (RX too, directional, whatever) pattern equal.

as recall, generally as you shorten dipoles L becomes less, or put another way overlength looks more L (inductive reactance), and shorter looks more C (capacitive reactance).

incidentally fatter dipoles have a wider bandwidth.

transition said:

if it’s an end-fed dipole the question’s redundant

but of folded or half-wave centre fed it keeps the radiation (RX too, directional, whatever) pattern equal.

as recall, generally as you shorten dipoles L becomes less, or put another way overlength looks more L (inductive reactance), and shorter looks more C (capacitive reactance).

incidentally fatter dipoles have a wider bandwidth.

and to complicate matter, there are antennas that are neither centre-fed, or end fed

transition said:

if it’s an end-fed dipole the question’s redundant

but of folded or half-wave centre fed it keeps the radiation (RX too, directional, whatever) pattern equal.

as recall, generally as you shorten dipoles L becomes less, or put another way overlength looks more L (inductive reactance), and shorter looks more C (capacitive reactance).

incidentally fatter dipoles have a wider bandwidth.

I have installed a dipole under the ceiling made of two lengths of copper pipe cut to 710mm for 105.50 mhz, I have soldered each side of the 300 ohm cable straight onto the copper, its receiving really well, To suspend it nder the ceiling, I screwed in an eyelet and threaded in a length of fishing wire through the two lengths of copper pipe to another eyelet. I plan to get a swr meter that can work on 105 mhz and swr the antenna in for more performance.

I have installed a dipole under the ceiling made of two lengths of copper pipe cut to 710mm for 105.50 mhz, I have soldered each side of the 300 ohm cable straight onto the copper, its receiving really well, To suspend it nder the ceiling, I screwed in an eyelet and threaded in a length of fishing wire through the two lengths of copper pipe to another eyelet. I plan to get a swr meter that can work on 105 mhz and swr the antenna in for more performance.

probably good enough. Keep ya balanced cable away from stuff some, and equal where it passes near.

all good.

transition said:

I have installed a dipole under the ceiling made of two lengths of copper pipe cut to 710mm for 105.50 mhz, I have soldered each side of the 300 ohm cable straight onto the copper, its receiving really well, To suspend it nder the ceiling, I screwed in an eyelet and threaded in a length of fishing wire through the two lengths of copper pipe to another eyelet. I plan to get a swr meter that can work on 105 mhz and swr the antenna in for more performance.

probably good enough. Keep ya balanced cable away from stuff some, and equal where it passes near.

all good.

>two lengths of copper pipe cut to 710mm

is that each 710mm, or together 710mm?

without thinking about it, a half wave should be ~710mm total?

what reckon?

transition said:

transition said:

I have installed a dipole under the ceiling made of two lengths of copper pipe cut to 710mm for 105.50 mhz, I have soldered each side of the 300 ohm cable straight onto the copper, its receiving really well, To suspend it nder the ceiling, I screwed in an eyelet and threaded in a length of fishing wire through the two lengths of copper pipe to another eyelet. I plan to get a swr meter that can work on 105 mhz and swr the antenna in for more performance.

probably good enough. Keep ya balanced cable away from stuff some, and equal where it passes near.

all good.

>two lengths of copper pipe cut to 710mm

is that each 710mm, or together 710mm?

without thinking about it, a half wave should be ~710mm total?

what reckon?

105 is 2.8m wavelength so 1/2 that is 1.4m.

I should know this stuff, and did about 30 odd years ago when I was working in the field. I took a turn into the power field in about ’87 and haven’t thought about this stuff since.

sibeen said:

I should know this stuff, and did about 30 odd years ago when I was working in the field. I took a turn into the power field in about ’87 and haven’t thought about this stuff since.

it’s ok, i’m here.

Bogsnorkler said:

sibeen said:

I should know this stuff, and did about 30 odd years ago when I was working in the field. I took a turn into the power field in about ’87 and haven’t thought about this stuff since.

it’s ok, i’m here.

Praise the lord :)

>105 is 2.8m wavelength so 1/2 that is 1.4m.

tempted to give you a gold star :-)

transition said:

>105 is 2.8m wavelength so 1/2 that is 1.4m.

tempted to give you a gold star :-)

Don’t.

Tau.Neutrino said:

transition said:

if it’s an end-fed dipole the question’s redundant

but of folded or half-wave centre fed it keeps the radiation (RX too, directional, whatever) pattern equal.

as recall, generally as you shorten dipoles L becomes less, or put another way overlength looks more L (inductive reactance), and shorter looks more C (capacitive reactance).

incidentally fatter dipoles have a wider bandwidth.

I plan to get a swr meter that can work on 105 mhz and swr the antenna in for more performance.

http://www.eham.net/ehamforum/smf/index.php?topic=32467.0

no point really.

Bogsnorkler said:

Tau.Neutrino said:

transition said:

if it’s an end-fed dipole the question’s redundant

but of folded or half-wave centre fed it keeps the radiation (RX too, directional, whatever) pattern equal.

as recall, generally as you shorten dipoles L becomes less, or put another way overlength looks more L (inductive reactance), and shorter looks more C (capacitive reactance).

incidentally fatter dipoles have a wider bandwidth.

I plan to get a swr meter that can work on 105 mhz and swr the antenna in for more performance.

http://www.eham.net/ehamforum/smf/index.php?topic=32467.0

no point really.

you would be better off putting a reflector and a director/s on the antenna.

sibeen said:

transition said:

>105 is 2.8m wavelength so 1/2 that is 1.4m.

tempted to give you a gold star :-)

Don’t.

That’s mean-spirited.

Here Bogsnorkler, have a gold star. Have several.