eHam

eHam Forums => Mobile Ham => Topic started by: AA4PB on July 06, 2004, 05:12:21 PM



Title: ICOM AH-4 Frequency Range?
Post by: AA4PB on July 06, 2004, 05:12:21 PM
Why? A 736 doesn't have 6 meters
--------------------------------
I think it does, Alan. At least the info I found on the Internet indicates it covers 6M and the original 706 covers 6M as I recall.


Title: ICOM AH-4 Frequency Range?
Post by: K2CMB on July 06, 2004, 12:03:09 AM
I thought the ICOM AH-4 was good to 50MHz, but not necessarily as a note in the "manual" indicates :

NOTE: 50 MHz tuning is possible with the IC-729*,
IC-706MKII, IC-746 and IC-756; the IC-706 and IC-
736 can only be tuned 3.5–30 MHz.

Why should this be, as 160m thru 6m share the same antenna port.  What could be different between the MKII and the original IC706 that might cause this limitation?


Title: ICOM AH-4 Frequency Range?
Post by: AA4PB on July 06, 2004, 08:47:35 AM
This is just a guess - perhaps those radios do not implement the tuner control functions (i.e. the TUNE button) on 6M.


Title: ICOM AH-4 Frequency Range?
Post by: K0BG on July 06, 2004, 09:34:25 AM
Why? A 736 doesn't have 6 meters.

The AH-4 will tune continuously from 1 Mhz through 60 Mhz with a caveat or two. The minimum length for 160 meter operation is about 45 to 50 feet if the unit has an adequate ground. On 75 meters the minimum length is about 25 feet again with an adequate ground. From 40 on up 8 to 9 feet is required.

The listed range is 5 to 5000 ohm, but in reality it is about 6.5 ohms to about 3500. While it will match high impedance loads up to 5000 ohms, the input SWR climbs because the output capacitors are fixed between 3500 and 5000 ohms.

Obviously, on certain frequencies using certain lengths, the coupler will not match as the antenna is too close to an exact half wave and the input impedance is out of the range of the tuner.

Alan, KØBG
www.k0bg.com


Title: ICOM AH-4 Frequency Range?
Post by: K0BG on July 07, 2004, 04:21:54 PM
I stand corrected Mr. Lweis. From looking at the partial manual I have, it appears the problem is as you suggested.

Alan, KØBG
www.k0bg.com


Title: ICOM AH-4 Frequency Range?
Post by: AD7DB on July 13, 2004, 02:43:33 PM
Strangely enough, I was very surprised to find that my AH-4 actually tunes my 8' mobile whip (actually 9' or so because of the wire from the tuner to the antenna mount) on 75m. It's not supposed to be able to do that but it does. Performance on there is crappy so I use my 75m Hamstick for that band. No luck on 160m yet, I guess I need more wire.


Title: ICOM AH-4 Frequency Range?
Post by: AA4PB on July 13, 2004, 06:19:54 PM
As you found out, the fact that a tuner can find a low SWR doesn't necessarily mean that it is an efficient match. Icom doesn't rate the AH-4 for an 8-foot whip on bands below 40M and it doesn't work well on 75M.

That rating doesn't apply to all tuners however. The SG-237 for example, IS rated to tune an 8-foot whip on 75M and it works as well as a 75M Hamstick.