Harvard 020 CB handheld - aerial length?

[SiriusHardware]

The Harvard 020 is a 1981 vintage 2-channel (14 and 30) low-power CB handheld - I've owned this example from new. Unlike many of the very cheap CB walkie talkies this is a 'proper' FM CB/27/81 set with a superhet receiver and the transmitter probably pushing out about 100mW. We got some pretty good distances (several miles, sometimes) when we tried it out originally.

Unfortunately, not long after I got it a friend accidentally snapped the original telescopic aerial which I remember being quite long.

In those days we had to settle for whatever could be physically obtained from nearby shops such as Tandy, or independent small electronics shops like Aitken Bros in Newcastle, or mail ordered from Maplin. The upshot was that the radio ended up with a rather shorter replacement aerial.

I picked it up again a few days ago and realised that if I wanted to, I could literally now search the whole world for a more suitable replacement aerial in a matter of moments: But the only question is: How long should it be?

Is there anyone here who still has one with its original aerial? If so, how long is it from the top of the bottom aerial section to the tip of the fully extended aerial?

[Restoration73]

The ideal for quarter wave is 2.7m, but as not possible use around 1m - there is an inductor in series with the aerial to make up the electrical length.
This might be OK (the longer the better !)
http://www.maplin.co.uk/p/10-section...y-aerial-lb10l

[Alistair D]

Somewhere in my loft I think I have a Realistic(Tandy) 2 channel hand held with a telescopic aerial. If no one has the same model as you I will dig mine out and measure it.

Al

[camtechman]

I've only recently started to collect 27/81 Handheld CB T/R's but I've only got one Havard model, the F007. It has a 9 section telescopic aerial and the fully extended length, from tip to base (where the securing screw is located) is 103cm (40.5").

[SiriusHardware]

Originally Posted by Restoration73

The ideal for quarter wave is 2.7m, but as not possible use around 1m - there is an inductor in series with the aerial to make up the electrical length.

Indeed there is, see attached diagram. The antenna coil T1 and the specific original length of the aerial will combine to produce what (as far as the radio is concerned) is a 27Mhz quarter wave. If the length of the aerial isn't right that combination is no longer optimal.

My guestimate - based vaguely on my memory (from more than 30 years ago!) is that the original aerial was somewhere between 80cm and 100cm. The length isn't the only factor though - the diameter of the bottom section has to be more or less exactly right so it is a snug fit in the exit hole, and the length of the collapsed aerial also has to be about right so that it is long enough to protrude through the top of the casing, but not so long that it sticks out too far even when fully collapsed.

Alastair D, thanks for the offer to measure the one on your Tandy model but the length of the aerial is usually directly related to the value of the in-series coil, which is likely to be different in every model. I really need the dimensions of an original Harvard 020 aerial.

Camtechman, nice example - bit of a cheesy model name, but not a bad looking set overall.

[vu2nan-nandu]

Originally Posted by SiriusHardware

Is there anyone here who still has one with its original aerial? If so, how long is it from the top of the bottom aerial section to the tip of the fully extended aerial?

Data from the Harvard -020 - User Manual

'Antenna length: 75cm, 7 sections, chromed'.

[SiriusHardware]

THANK you for that - I don't know where my original manual is these days but it would never have occurred to me to look in there for that information.

There remains, however, the question of whether that dimension means 75cm from the top of the unit to the tip of the fully extended antenna, or whether it includes the 'hidden' bottom section inside the unit.

(Common sense would suggest they meant the length from the top of the body to the tip of the antenna?)

[G6Tanuki]

I guess it means the length of the antenna protruding outside the radio when fully extended.

Does it have a separate external-antenna-socket? if so, always make sure that if you're using an antenna in the socket that the telescopic whip's fully retracted, otherwise you can get some serious mismatch on the final-output transistor.

This was a problem with some amateur 2-metre VHF portable radios - the better ones had a switch fitted to the bottom of the telescopic whip which - when the whip was fully retracted - disconnected it and switched the RF output directly to the external-antenna socket only.

[Alistair D]

If you can measure the value of the loading coil this calculator may help decide the correct length

http://www.daycounter.com/Calculator...lculator.phtml

Al

[SiriusHardware]

Originally Posted by G6Tanuki

I guess it means the length of the antenna protruding outside the radio when fully extended.

Does it have a separate external-antenna-socket? if so, always make sure that if you're using an antenna in the socket that the telescopic whip's fully retracted, otherwise you can get some serious mismatch on the final-output transistor.

No, it's a basic two-channel crystal controlled set and does not have an external aerial socket. (See diagram in post # 5). However, it does have a 'proper' three-stage transmitter so it could just as easily be damaged by NOT using the aerial fully extended or with an aerial which is not the designed-for length, hence my interest in trying to restore the antenna to original length.

I also own a full-featured Realistic TRC1001 handset which does use the scheme you mentioned, the telescopic aerial is not switched out when an external aerial is in use - but when fully retracted the match is so bad that it does not 'exist' as far as the output stage is concerned.

[SiriusHardware]

Originally Posted by Alistair D

If you can measure the value of the loading coil this calculator may help decide the correct length

A good idea (and a good lead, thanks for that), but I don't have an LCR meter. A good excuse to finally buy one, perhaps.

The set is at work at the moment - I'm trying to remember whether the antenna coil (T1) is a tunable one, ie, typical Toko tin-can type.

If so, I could deliberately fit an over-length telescopic aerial temporarily and shorten it gradually until the RF output 'peak' from it according to an S/RF meter coincides with where the coil is set to now. i.e,

-Note exactly where the coil core is set now by some arbitrary method (count the number of quarter turns in from flush with the top of the coil, or something like that).

-Set the aerial to about the right length (75cm protruding from the top).

-Adjust the aerial coil in and out a little (with a non-metallic tool, of course) to see where the output peaks. Adjust the length of the aerial a little at a time one way or the other, then re-peak the coil. When peak output is obtained with the coil set to where it was originally set, the aerial must be the same length as the original aerial was...

Does that make sense?

[vu2nan-nandu]

Originally Posted by SiriusHardware

There remains, however, the question of whether that dimension means 75cm from the top of the unit to the tip of the fully extended antenna, or whether it includes the 'hidden' bottom section inside the unit.

It's appears to be a description of the antenna itself (7 sections, chromed). Hence 75cm would be the total length.

[SiriusHardware]

Nandu, thanks for the clarification, but that does puzzle me because the aerial on the set now has a total length of 70cms including the section inside the radio: But I remember the original antenna seeming significantly longer (not just 5cm longer, but quite a bit longer).

Perhaps the way forward is to non-destructively add about 20cm to the existing short aerial, then gradually shorten it until peak indicated output coincides with the original setting of the core in the antenna coil. At that point, I can hopefully assume that the adjusted length of the temporarily extended, then shortened aerial is the same as the original aerial.

[vu2nan-nandu]

You're right, Sirius. The original length would be the one at which you get maximum output on transmit / maximum signal strength on receive keeping the core of the loading coil in the original position. Have fun!