Optimal Arial Lenghts for TX

[ICEMAN]

The "ATC" tower where im working is using an Icom a6 handheld radio, with about 3 meters of co-ax cable used to connect to a whip arial which is mounted about 5 meters above the ground.

We are able to recieve inbound A/C from about 58 miles, however can only Tx with reasonable clarity for about 10-12 miles ie on finals..

Im pretty sure that the whip arial is too long for the handheld unit, can any one advise what the optimal lenght should be for operations on 118,5.

In addition, what is the general lenght for a 1/4wave whip arial? (i know it covers most aeronautical frequency ranges)

Stan

[Wart]

118.5 MHz has a wavelength of 9.4506 meters. Therefore 1/4 wavelength = 2.362 meters.
This 2.362 meters does not have to be one single length and can be matched (using coils and caps) to a shorter lenghth of "wire" or to a helical type whip antenna, as is usually supplied. Your best transmission and reception will be acheived with a full length antenna or a 1/4 wave dipole, but these can be directional.

A hand-held A6 has an output power of 5W (PEP) and a transmit distance of 10 - 12 miles is acceptable (usually line of sight). The receive clarity and distance is a function of the transmitting station i.e. the aeries radio, and not that of the A6.

I would check that you have a sufficient enough ground-plane on the current antenna as this wil affect its performance.

[kb]

Also, to work out the theoretical distance of RT, do the following equation:
1.25 x square root of transmitter hight (AGL) + 1.25 x square root reciever hight (AGL. This is the range in NM.
Also, this is theoritical. Sometimes the TX power will not allow this.

unnerstend??

[ICEMAN]

unnerstend??

... JA, dus i goot, danke


Thanks kb and wart

[John Young]

118.5 MHz has a wavelength of 9.4506 meters. Therefore 1/4 wavelength = 2.362 meters.

Wart, nearly but not quite right.

Wavelength is calculated as: -

A factor of 300, divided by frequency in MHz, multiplied by 0.93 = wavelength in metres. Therefore, in terms of Iceman's question -

"Can anyone advise what the optimum antenna length should be for operation on 118.5?"

300 divided by 118.5 X 0.93 = 2,354 meters (quarter wave = 588.6 mm).

Glad that I'm a ham radio operator ZS5WG - 73's to all.

[Wart]

OOOPPPSSS!!!!

Wavelength = c (speed of light in a vacuum) DIVIDED BY f (frequency),

c = 300 000 km/s = 300 000 000 m/s
f = 118.5 MHz

Therefore, Wavelength = 300 / 118.5 = 2.5316 meters
1/4 Wave = 633 mm (approx.)

Q: Why do you use a factor of 0.93 ?

[John Young]

Q: Why do you use a factor of 0.93 ?

Multiply the result by 0.93 to compensate for metal (versus air).

Therefore, corrected by 0.93 gives the final result of a tuned length of 588.6 mm in the above example.

73's de John ZS5WG

EISH !! Remind me not to get kb to tune my antenna !!

[Morph]

EISH !! Remind me not to get kb to tune my antenna !!

I'll choon your antenna for you, "hoesit my bra, howzit standing "

Funny enough, from all my years in Telkom, which included me teaching at Telkom College in Olifantsfontein, they/we/I never taught any compensation for metal vs air. Maybe this is why the old international radio calls were so unreliable.

[ICEMAN]

So am i correcct in finalising on a lenght of: 588.6 mm as optimal

Seems like i inavertantly started another bun fight ......... sorry gents !!

[Morph]

John's posted an Excel Spreadsheet to use when determining antenna length

http://www.gregperkins.co.za/flying/dow ... ulator.xls

[ICEMAN]

Thank you John..... very nice attachment.

Question: Full lenght (2,35443m) vs 1/4 wave (588,6mm), any prefrences or do they work equally well

[Morph]

The full length antenna, though not always feasible will be the best.

[ICEMAN]

Thx Morph, as its a portable ground based station, the full lenght should be feasible without a problem..... tx to all for the feedback

Stan

[John Young]

So am i correcct in finalising on a lenght of: 588.6 mm as optimal

Yes Iceman, you would be quite correct at 588.6 mm.

But came and “tell” how will you measure it? From where to where? Now that is the question.

OK, in simple terms. A radio will transmit maximum power PEP [Peak Envelope Power] through the antenna only when the antenna is matched [cut to length] for the frequency being used. Here, assume that an antenna tuner is not going to be used.

Right, now we introduce SWR [Standing Wave Ratio]. SWR simplified, “What proportion of your TX power is being “reflected back at the radio” because of a poor antenna match”?

A properly matched antenna will give a reading of 1:1 on a SWR meter. A poorly matched antenna will give a reading > 3:1. This is important when using high power being > 100 Watts as the radio “finals” could be damaged. Most certainly, damage would occur at > 400 W PEP.

We M/L’s generally use between 1 to 5 Watts which is more than adequate and our radios will perform as designed with a SWR reading < 3:1.

You will notice that most M/L quarter wave antenna mounts incorporate a hole into which the antenna is inserted. The antenna is held in place with a grub screw. The SWR is adjusted with the aid of a SWR meter by loosening the grub screw and moving the antenna to different depths in the mount [adjusting the length]. Alternatively, and depending the antenna match, insert the antenna fully and then, using side-cutters, trim 2 mm off the antenna at a time until optimum length is reached.

But best still, get a ham radio mate to come out to the field – line up the M/L’s, and walla, good comms all round.

73’s de John ZS5WG

EISH !! I see more questions - I will revert just now.

[IFLYHI]

How do I measure this My whip is mounted on a base with a little spring point, is it from the pivot point or from the little grub screw holding the whip.

Do I assume correctly, that to cover all bases, 555mm seems a good average for the arial